Added explanation comment for excluding files.

src/services/services.ts

@@ -1834,7 +1834,8 @@ namespace ts {
             const fileContents = sourceFile.text;
             const result: TodoComment[] = [];
 
-            if (descriptors.length > 0 && !isNodeModulesFile(fileName)) {
+            // Exclude node_modules files as we don't want to show the todos of external libraries.
+            if (descriptors.length > 0 && !isNodeModulesFile(sourceFile.fileName)) {
                 const regExp = getTodoCommentsRegExp();
 
                 let matchArray: RegExpExecArray;

tests/cases/fourslash/todoComments18.ts

@@ -1,6 +1,8 @@
+/// <reference path='fourslash.ts' />
+
 // Tests node_modules name in file still gets todos.
 
-// @Filename: /node_modules_todotest0.ts
+// @Filename: /node_modules_todoTest0.ts
 //// // [|TODO|]
 
 verify.todoCommentsInCurrentFile(["TODO"]);

tests/cases/fourslash/todoComments19.ts

@@ -1,6 +1,11 @@
+/// <reference path='fourslash.ts' />
+
 // Tests that todos are not found in node_modules folder.
 
-// @Filename: /node_modules/todotest0.ts
+// @Filename: todoTest0.ts
+//// import * as foo1 from "fake-module";
+
+// @Filename: node_modules/fake-module/ts.ts
 //// // TODO
 
 verify.todoCommentsInCurrentFile(["TODO"]);

tests/cases/fourslash/todoComments20.ts

@@ -1,4 +1,6 @@
-// @Filename: dir1/node_modules/todotest0.ts
+/// <reference path='fourslash.ts' />
+
+// @Filename: dir1/node_modules/todoTest0.ts
 //// // TODO
 
 verify.todoCommentsInCurrentFile(["TODO"]);