diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index f76b0a39fb..e9022bff0d 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -3685,6 +3685,10 @@
         "category": "Error",
         "code": 6369
     },
+    "Skipping clean because not all projects could be located": {
+        "category": "Error",
+        "code": 6340
+    },
 
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
diff --git a/src/compiler/tsbuild.ts b/src/compiler/tsbuild.ts
index 2a509a8c74..1f36f09393 100644
--- a/src/compiler/tsbuild.ts
+++ b/src/compiler/tsbuild.ts
@@ -38,6 +38,10 @@ namespace ts {
          * Issue a verbose diagnostic message. No-ops when options.verbose is false.
          */
         verbose(diag: DiagnosticMessage, ...args: any[]): void;
+
+        invalidatedProjects: FileMap<true>;
+        queuedProjects: FileMap<true>;
+        missingRoots: Map<true>;
     }
 
     type Mapper = ReturnType<typeof createDependencyMapper>;
@@ -73,7 +77,7 @@ namespace ts {
         AnyErrors = ConfigFileErrors | SyntaxErrors | TypeErrors | DeclarationEmitErrors
     }
 
-    enum UpToDateStatusType {
+    export enum UpToDateStatusType {
         Unbuildable,
         UpToDate,
         /**
@@ -89,7 +93,7 @@ namespace ts {
         UpstreamBlocked
     }
 
-    type UpToDateStatus =
+    export type UpToDateStatus =
         | Status.Unbuildable
         | Status.UpToDate
         | Status.OutputMissing
@@ -98,7 +102,7 @@ namespace ts {
         | Status.UpstreamOutOfDate
         | Status.UpstreamBlocked;
 
-    namespace Status {
+    export namespace Status {
         /**
          * The project can't be built at all in its current state. For example,
          * its config file cannot be parsed, or it has a syntax error or missing file
@@ -170,6 +174,9 @@ namespace ts {
         setValue(fileName: string, value: T): void;
         getValue(fileName: string): T | never;
         getValueOrUndefined(fileName: string): T | undefined;
+        hasKey(fileName: string): boolean;
+        removeKey(fileName: string): void;
+        getKeys(): string[];
     }
 
     /**
@@ -183,8 +190,23 @@ namespace ts {
             setValue,
             getValue,
             getValueOrUndefined,
+            removeKey,
+            getKeys,
+            hasKey
         };
 
+        function getKeys(): string[] {
+            return Object.keys(lookup);
+        }
+
+        function hasKey(fileName: string) {
+            return normalizePath(fileName) in lookup;
+        }
+
+        function removeKey(fileName: string) {
+            delete lookup[fileName];
+        }
+
         function setValue(fileName: string, value: T) {
             lookup[normalizePath(fileName)] = value;
         }
@@ -211,30 +233,30 @@ namespace ts {
     }
 
     export function createDependencyMapper() {
-        const childToParents: { [key: string]: string[] } = {};
-        const parentToChildren: { [key: string]: string[] } = {};
-        const allKeys: string[] = [];
+        const childToParents: { [key: string]: ResolvedConfigFileName[] } = {};
+        const parentToChildren: { [key: string]: ResolvedConfigFileName[] } = {};
+        const allKeys: ResolvedConfigFileName[] = [];
 
-        function addReference(childConfigFileName: string, parentConfigFileName: string): void {
+        function addReference(childConfigFileName: ResolvedConfigFileName, parentConfigFileName: ResolvedConfigFileName): void {
             addEntry(childToParents, childConfigFileName, parentConfigFileName);
             addEntry(parentToChildren, parentConfigFileName, childConfigFileName);
         }
 
-        function getReferencesTo(parentConfigFileName: string): string[] {
+        function getReferencesTo(parentConfigFileName: ResolvedConfigFileName): ResolvedConfigFileName[] {
             return parentToChildren[normalizePath(parentConfigFileName)] || [];
         }
 
-        function getReferencesOf(childConfigFileName: string): string[] {
+        function getReferencesOf(childConfigFileName: ResolvedConfigFileName): ResolvedConfigFileName[] {
             return childToParents[normalizePath(childConfigFileName)] || [];
         }
 
-        function getKeys(): ReadonlyArray<string> {
+        function getKeys(): ReadonlyArray<ResolvedConfigFileName> {
             return allKeys;
         }
 
-        function addEntry(mapToAddTo: typeof childToParents | typeof parentToChildren, key: string, element: string) {
-            key = normalizePath(key);
-            element = normalizePath(element);
+        function addEntry(mapToAddTo: typeof childToParents | typeof parentToChildren, key: ResolvedConfigFileName, element: ResolvedConfigFileName) {
+            key = normalizePath(key) as ResolvedConfigFileName;
+            element = normalizePath(element) as ResolvedConfigFileName;
             const arr = (mapToAddTo[key] = mapToAddTo[key] || []);
             if (arr.indexOf(element) < 0) {
                 arr.push(element);
@@ -316,8 +338,13 @@ namespace ts {
             return parsed;
         }
 
+        function removeKey(configFilePath: ResolvedConfigFileName) {
+            cache.removeKey(configFilePath);
+        }
+
         return {
-            parseConfigFile
+            parseConfigFile,
+            removeKey
         };
     }
 
@@ -331,11 +358,19 @@ namespace ts {
 
     export function createBuildContext(options: BuildOptions, reportDiagnostic: DiagnosticReporter): BuildContext {
         const verboseDiag = options.verbose && reportDiagnostic;
+
+        const invalidatedProjects = createFileMap<true>();
+        const queuedProjects = createFileMap<true>();
+        const missingRoots = createMap<true>();
+
         return {
             options,
             projectStatus: createFileMap(),
             unchangedOutputs: createFileMap(),
-            verbose: verboseDiag ? (diag, ...args) => verboseDiag(createCompilerDiagnostic(diag, ...args)) : () => undefined
+            verbose: verboseDiag ? (diag, ...args) => verboseDiag(createCompilerDiagnostic(diag, ...args)) : () => undefined,
+            invalidatedProjects,
+            missingRoots,
+            queuedProjects
         };
     }
 
@@ -437,12 +472,12 @@ namespace ts {
             addProject(".");
         }
 
-        const builder = createSolutionBuilder(host, reportDiagnostic, { verbose, dry, force });
+        const builder = createSolutionBuilder(host, projects, reportDiagnostic, { verbose, dry, force });
         if (clean) {
-            builder.cleanProjects(projects);
+            builder.cleanAllProjects();
         }
         else {
-            builder.buildProjects(projects);
+            builder.buildAllProjects();
         }
 
         function addProject(projectSpecification: string) {
@@ -461,7 +496,11 @@ namespace ts {
         }
     }
 
-    export function createSolutionBuilder(host: CompilerHost, reportDiagnostic: DiagnosticReporter, defaultOptions: BuildOptions) {
+    /**
+     * A SolutionBuilder has an immutable set of rootNames that are the "entry point" projects, but
+     * can dynamically add/remove other projects based on changes on the rootNames' references
+     */
+    export function createSolutionBuilder(host: CompilerHost, rootNames: ReadonlyArray<string>, reportDiagnostic: DiagnosticReporter, defaultOptions: BuildOptions) {
         if (!host.getModifiedTime || !host.setModifiedTime) {
             throw new Error("Host must support timestamp APIs");
         }
@@ -470,12 +509,18 @@ namespace ts {
         let context = createBuildContext(defaultOptions, reportDiagnostic);
 
         return {
+            buildAllProjects,
             getUpToDateStatus,
             getUpToDateStatusOfFile,
-            buildProjects,
-            cleanProjects,
+            cleanAllProjects,
             resetBuildContext,
-            getBuildGraph
+            getBuildGraph,
+
+            invalidateProject,
+            buildInvalidatedProjects,
+            buildDependentInvalidatedProjects,
+
+            resolveProjectName
         };
 
         function resetBuildContext(opts = defaultOptions) {
@@ -486,13 +531,17 @@ namespace ts {
             return getUpToDateStatus(configFileCache.parseConfigFile(configFileName));
         }
 
-        function getBuildGraph(configFileNames: string[]) {
+        function getBuildGraph(configFileNames: ReadonlyArray<string>) {
             const resolvedNames: ResolvedConfigFileName[] | undefined = resolveProjectNames(configFileNames);
-            if (resolvedNames === undefined) return;
+            if (resolvedNames === undefined) return undefined;
 
             return createDependencyGraph(resolvedNames);
         }
 
+        function getGlobalDependencyGraph() {
+            return getBuildGraph(rootNames);
+        }
+
         function getUpToDateStatus(project: ParsedCommandLine | undefined): UpToDateStatus {
             if (project === undefined) {
                 return { type: UpToDateStatusType.Unbuildable, reason: "File deleted mid-build" };
@@ -507,6 +556,73 @@ namespace ts {
             return actual;
         }
 
+        function invalidateProject(configFileName: string) {
+            const resolved = resolveProjectName(configFileName);
+            if (resolved === undefined) {
+                // If this was a rootName, we need to track it as missing.
+                // Otherwise we can just ignore it and have it possibly surface as an error in any downstream projects,
+                // if they exist
+
+                // TODO: do those things
+                return;
+            }
+
+            configFileCache.removeKey(resolved);
+            context.invalidatedProjects.setValue(resolved, true);
+            context.projectStatus.removeKey(resolved);
+
+            const graph = getGlobalDependencyGraph()!;
+            if (graph) {
+                queueBuildForDownstreamReferences(resolved);
+            }
+
+            // Mark all downstream projects of this one needing to be built "later"
+            function queueBuildForDownstreamReferences(root: ResolvedConfigFileName) {
+                debugger;
+                const deps = graph.dependencyMap.getReferencesTo(root);
+                for (const ref of deps) {
+                    // Can skip circular references
+                    if (!context.queuedProjects.hasKey(ref)) {
+                        context.queuedProjects.setValue(ref, true);
+                        queueBuildForDownstreamReferences(ref);
+                    }
+                }
+            }
+        }
+
+        function buildInvalidatedProjects() {
+            buildSomeProjects(p => context.invalidatedProjects.hasKey(p));
+        }
+
+        function buildDependentInvalidatedProjects() {
+            buildSomeProjects(p => context.queuedProjects.hasKey(p));
+        }
+
+        function buildSomeProjects(predicate: (projName: ResolvedConfigFileName) => boolean) {
+            const resolvedNames: ResolvedConfigFileName[] | undefined = resolveProjectNames(rootNames);
+            if (resolvedNames === undefined) return;
+
+            const graph = createDependencyGraph(resolvedNames)!;
+            for (const next of graph.buildQueue) {
+                if (!predicate(next)) continue;
+
+                const resolved = resolveProjectName(next);
+                if (!resolved) continue; // ??
+                const proj = configFileCache.parseConfigFile(resolved);
+                if (!proj) continue; // ?
+
+                const status = getUpToDateStatus(proj);
+                reportProjectStatus(next, status);
+
+                if (status.type === UpToDateStatusType.UpstreamBlocked) {
+                    context.verbose(Diagnostics.Skipping_build_of_project_0_because_its_upstream_project_1_has_errors, resolved, status.upstreamProjectName);
+                    continue;
+                }
+
+                buildSingleProject(next);
+            }
+        }
+
         function getAllProjectOutputs(project: ParsedCommandLine): ReadonlyArray<string> {
             if (project.options.outFile) {
                 return getOutFileOutputs(project);
@@ -824,7 +940,7 @@ namespace ts {
             context.projectStatus.setValue(proj.options.configFilePath!, { type: UpToDateStatusType.UpToDate, newestDeclarationFileContentChangedTime: priorNewestUpdateTime } as UpToDateStatus);
         }
 
-        function getFilesToClean(configFileNames: ResolvedConfigFileName[]): string[] | undefined {
+        function getFilesToClean(configFileNames: ReadonlyArray<ResolvedConfigFileName>): string[] | undefined {
             const resolvedNames: ResolvedConfigFileName[] | undefined = resolveProjectNames(configFileNames);
             if (resolvedNames === undefined) return undefined;
 
@@ -849,12 +965,24 @@ namespace ts {
             return filesToDelete;
         }
 
-        function cleanProjects(configFileNames: string[]) {
-            const resolvedNames: ResolvedConfigFileName[] | undefined = resolveProjectNames(configFileNames);
-            if (resolvedNames === undefined) return;
+        function getAllProjectsInScope(): ReadonlyArray<ResolvedConfigFileName> | undefined {
+            const resolvedNames = resolveProjectNames(rootNames);
+            if (resolvedNames === undefined) return undefined;
+            const graph = createDependencyGraph(resolvedNames);
+            if (graph === undefined) return undefined;
+            return graph.buildQueue;
+        }
+
+        function cleanAllProjects() {
+            const resolvedNames: ReadonlyArray<ResolvedConfigFileName> | undefined = getAllProjectsInScope();
+            if (resolvedNames === undefined) {
+                reportDiagnostic(createCompilerDiagnostic(Diagnostics.Skipping_clean_because_not_all_projects_could_be_located));
+                return;
+            }
 
             const filesToDelete = getFilesToClean(resolvedNames);
             if (filesToDelete === undefined) {
+                reportDiagnostic(createCompilerDiagnostic(Diagnostics.Skipping_clean_because_not_all_projects_could_be_located));
                 return;
             }
 
@@ -885,7 +1013,7 @@ namespace ts {
             return undefined;
         }
 
-        function resolveProjectNames(configFileNames: string[]): ResolvedConfigFileName[] | undefined {
+        function resolveProjectNames(configFileNames: ReadonlyArray<string>): ResolvedConfigFileName[] | undefined {
             const resolvedNames: ResolvedConfigFileName[] = [];
             for (const name of configFileNames) {
                 const resolved = resolveProjectName(name);
@@ -897,12 +1025,8 @@ namespace ts {
             return resolvedNames;
         }
 
-        function buildProjects(configFileNames: string[]) {
-            const resolvedNames: ResolvedConfigFileName[] | undefined = resolveProjectNames(configFileNames);
-            if (resolvedNames === undefined) return;
-
-            // Establish what needs to be built
-            const graph = createDependencyGraph(resolvedNames);
+        function buildAllProjects() {
+            const graph = getGlobalDependencyGraph();
             if (graph === undefined) return;
 
             const queue = graph.buildQueue;
diff --git a/src/harness/unittests/tsbuild.ts b/src/harness/unittests/tsbuild.ts
index ed1fd96795..46fee63a7c 100644
--- a/src/harness/unittests/tsbuild.ts
+++ b/src/harness/unittests/tsbuild.ts
@@ -19,11 +19,10 @@ namespace ts {
         it("can build the sample project 'sample1' without error", () => {
             const fs = bfs.shadow();
             const host = new fakes.CompilerHost(fs);
-            const builder = createSolutionBuilder(host, reportDiagnostic, { dry: false, force: false, verbose: false });
+            const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: false, verbose: false });
 
             clearDiagnostics();
-            fs.chdir("/src/tests");
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
             assertDiagnosticMessages(/*empty*/);
 
             // Check for outputs. Not an exhaustive list
@@ -38,9 +37,8 @@ namespace ts {
             clearDiagnostics();
             const fs = bfs.shadow();
             const host = new fakes.CompilerHost(fs);
-            const builder = createSolutionBuilder(host, reportDiagnostic, { dry: true, force: false, verbose: false });
-            fs.chdir("/src/tests");
-            builder.buildProjects(["."]);
+            const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: true, force: false, verbose: false });
+            builder.buildAllProjects();
             assertDiagnosticMessages(Diagnostics.Would_build_project_0, Diagnostics.Would_build_project_0, Diagnostics.Would_build_project_0);
 
             // Check for outputs to not be written. Not an exhaustive list
@@ -54,14 +52,13 @@ namespace ts {
             const fs = bfs.shadow();
             const host = new fakes.CompilerHost(fs);
 
-            let builder = createSolutionBuilder(host, reportDiagnostic, { dry: false, force: false, verbose: false });
-            fs.chdir("/src/tests");
-            builder.buildProjects(["."]);
+            let builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: false, verbose: false });
+            builder.buildAllProjects();
             tick();
 
             clearDiagnostics();
-            builder = createSolutionBuilder(host, reportDiagnostic, { dry: true, force: false, verbose: false });
-            builder.buildProjects(["."]);
+            builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: true, force: false, verbose: false });
+            builder.buildAllProjects();
             assertDiagnosticMessages(Diagnostics.Project_0_is_up_to_date, Diagnostics.Project_0_is_up_to_date, Diagnostics.Project_0_is_up_to_date);
         });
     });
@@ -72,20 +69,19 @@ namespace ts {
             const fs = bfs.shadow();
             const host = new fakes.CompilerHost(fs);
 
-            const builder = createSolutionBuilder(host, reportDiagnostic, { dry: false, force: false, verbose: false });
-            fs.chdir("/src/tests");
-            builder.buildProjects(["."]);
+            const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: false, verbose: false });
+            builder.buildAllProjects();
             // Verify they exist
             for (const output of allExpectedOutputs) {
                 assert(fs.existsSync(output), `Expect file ${output} to exist`);
             }
-            builder.cleanProjects(["."]);
+            builder.cleanAllProjects();
             // Verify they are gone
             for (const output of allExpectedOutputs) {
                 assert(!fs.existsSync(output), `Expect file ${output} to not exist`);
             }
             // Subsequent clean shouldn't throw / etc
-            builder.cleanProjects(["."]);
+            builder.cleanAllProjects();
         });
     });
 
@@ -94,16 +90,15 @@ namespace ts {
             const fs = bfs.shadow();
             const host = new fakes.CompilerHost(fs);
 
-            const builder = createSolutionBuilder(host, reportDiagnostic, { dry: false, force: true, verbose: false });
-            fs.chdir("/src/tests");
-            builder.buildProjects(["."]);
+            const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: true, verbose: false });
+            builder.buildAllProjects();
             let currentTime = time();
             checkOutputTimestamps(currentTime);
 
             tick();
             Debug.assert(time() !== currentTime, "Time moves on");
             currentTime = time();
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
             checkOutputTimestamps(currentTime);
 
             function checkOutputTimestamps(expected: number) {
@@ -119,14 +114,12 @@ namespace ts {
     describe("tsbuild - can detect when and what to rebuild", () => {
         const fs = bfs.shadow();
         const host = new fakes.CompilerHost(fs);
-        const builder = createSolutionBuilder(host, reportDiagnostic, { dry: false, force: false, verbose: true });
-
-        fs.chdir("/src/tests");
+        const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: false, verbose: true });
 
         it("Builds the project", () => {
             clearDiagnostics();
             builder.resetBuildContext();
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
             assertDiagnosticMessages(Diagnostics.Sorted_list_of_input_projects_Colon_0,
                 Diagnostics.Project_0_is_out_of_date_because_output_file_1_does_not_exist,
                 Diagnostics.Building_project_0,
@@ -141,7 +134,7 @@ namespace ts {
         it("Detects that all projects are up to date", () => {
             clearDiagnostics();
             builder.resetBuildContext();
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
             assertDiagnosticMessages(Diagnostics.Sorted_list_of_input_projects_Colon_0,
                 Diagnostics.Project_0_is_up_to_date_because_newest_input_1_is_older_than_oldest_output_2,
                 Diagnostics.Project_0_is_up_to_date_because_newest_input_1_is_older_than_oldest_output_2,
@@ -154,7 +147,7 @@ namespace ts {
             clearDiagnostics();
             fs.writeFileSync("/src/tests/index.ts", "const m = 10;");
             builder.resetBuildContext();
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
 
             assertDiagnosticMessages(Diagnostics.Sorted_list_of_input_projects_Colon_0,
                 Diagnostics.Project_0_is_up_to_date_because_newest_input_1_is_older_than_oldest_output_2,
@@ -169,7 +162,7 @@ namespace ts {
             clearDiagnostics();
             replaceText(fs, "/src/core/index.ts", "HELLO WORLD", "WELCOME PLANET");
             builder.resetBuildContext();
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
 
             assertDiagnosticMessages(Diagnostics.Sorted_list_of_input_projects_Colon_0,
                 Diagnostics.Project_0_is_out_of_date_because_oldest_output_1_is_older_than_newest_input_2,
@@ -185,14 +178,13 @@ namespace ts {
         it("won't build downstream projects if upstream projects have errors", () => {
             const fs = bfs.shadow();
             const host = new fakes.CompilerHost(fs);
-            const builder = createSolutionBuilder(host, reportDiagnostic, { dry: false, force: false, verbose: true });
+            const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: false, verbose: true });
 
             clearDiagnostics();
 
             // Induce an error in the middle project
             replaceText(fs, "/src/logic/index.ts", "c.multiply(10, 15)", `c.muitply()`);
-            fs.chdir("/src/tests");
-            builder.buildProjects(["."]);
+            builder.buildAllProjects();
             assertDiagnosticMessages(
                 Diagnostics.Sorted_list_of_input_projects_Colon_0,
                 Diagnostics.Project_0_is_out_of_date_because_output_file_1_does_not_exist,
@@ -221,8 +213,6 @@ namespace ts {
 
         writeProjects(fs, ["A", "B", "C", "D", "E", "F", "G"], deps);
 
-        const builder = createSolutionBuilder(host, reportDiagnostic, { dry: true, force: false, verbose: false });
-
         it("orders the graph correctly - specify two roots", () => {
             checkGraphOrdering(["A", "G"], ["A", "B", "C", "D", "E", "G"]);
         });
@@ -240,6 +230,8 @@ namespace ts {
         });
 
         function checkGraphOrdering(rootNames: string[], expectedBuildSet: string[]) {
+            const builder = createSolutionBuilder(host, rootNames, reportDiagnostic, { dry: true, force: false, verbose: false });
+
             const projFileNames = rootNames.map(getProjectFileName);
             const graph = builder.getBuildGraph(projFileNames);
             if (graph === undefined) throw new Error("Graph shouldn't be undefined");
@@ -280,6 +272,39 @@ namespace ts {
         }
     });
 
+    describe("tsbuild - project invalidation", () => {
+        it ("invalidates projects correctly", () => {
+            const fs = bfs.shadow();
+            const host = new fakes.CompilerHost(fs);
+            const builder = createSolutionBuilder(host, ["/src/tests"], reportDiagnostic, { dry: false, force: false, verbose: false });
+
+            clearDiagnostics();
+            builder.buildAllProjects();
+            assertDiagnosticMessages(/*empty*/);
+
+            // Update a timestamp in the middle project
+            tick();
+            touch(fs, "/src/logic/index.ts");
+            // Because we haven't reset the build context, the builder should assume there's nothing to do right now
+            const status = builder.getUpToDateStatusOfFile(builder.resolveProjectName("/src/logic")!);
+            assert.equal(status.type, UpToDateStatusType.UpToDate, "Project should be assumed to be up-to-date");
+
+            // Rebuild this project
+            tick();
+            builder.invalidateProject("/src/logic");
+            builder.buildInvalidatedProjects();
+            // The file should be updated
+            assert.equal(fs.statSync("/src/logic/index.js").mtimeMs, time(), "JS file should have been rebuilt");
+            assert.isBelow(fs.statSync("/src/tests/index.js").mtimeMs, time(), "Downstream JS file should *not* have been rebuilt");
+
+            // Build downstream projects should update 'tests', but not 'core'
+            tick();
+            builder.buildDependentInvalidatedProjects();
+            assert.equal(fs.statSync("/src/tests/index.js").mtimeMs, time(), "Downstream JS file should have been rebuilt");
+            assert.isBelow(fs.statSync("/src/core/index.js").mtimeMs, time(), "Upstream JS file should not have been rebuilt");
+        });
+    });
+
     function replaceText(fs: vfs.FileSystem, path: string, oldText: string, newText: string) {
         if (!fs.statSync(path).isFile()) {
             throw new Error(`File ${path} does not exist`);
@@ -324,6 +349,13 @@ namespace ts {
         return currentTime;
     }
 
+    function touch(fs: vfs.FileSystem, path: string) {
+        if (!fs.statSync(path).isFile()) {
+            throw new Error(`File ${path} does not exist`);
+        }
+        fs.utimesSync(path, new Date(time()), new Date(time()));
+    }
+
     function loadFsMirror(vfs: vfs.FileSystem, localRoot: string, virtualRoot: string) {
         vfs.mkdirpSync(virtualRoot);
         for (const path of Harness.IO.readDirectory(localRoot)) {