Add test case for reporting file preprocessing error in javascript file

src/compiler/program.ts

@@ -1077,6 +1077,8 @@ namespace ts {
                         const importedFile = findSourceFile(resolution.resolvedFileName, toPath(resolution.resolvedFileName, currentDirectory, getCanonicalFileName), /*isDefaultLib*/ false, file, skipTrivia(file.text, file.imports[i].pos), file.imports[i].end);
 
                         if (importedFile && resolution.isExternalLibraryImport) {
+                            // Since currently irrespective of allowJs, we only look for supportedTypeScript extension external module files,
+                            // this check is ok. Otherwise this would be never true for javascript file
                             if (!isExternalModule(importedFile)) {
                                 const start = getTokenPosOfNode(file.imports[i], file);
                                 fileProcessingDiagnostics.add(createFileDiagnostic(file, start, file.imports[i].end - start, Diagnostics.Exported_external_package_typings_file_0_is_not_a_module_Please_contact_the_package_author_to_update_the_package_definition, importedFile.fileName));

tests/baselines/reference/jsFileCompilationExternalPackageError.errors.txt

@@ -0,0 +1,19 @@
+tests/cases/compiler/moduleA/a.js(2,17): error TS2656: Exported external package typings file 'tests/cases/compiler/node_modules/b.ts' is not a module. Please contact the package author to update the package definition.
+
+
+==== tests/cases/compiler/moduleA/a.js (1 errors) ====
+    
+    import {a} from "b";
+                    ~~~
+!!! error TS2656: Exported external package typings file 'b.ts' is not a module. Please contact the package author to update the package definition.
+    a++;
+    import {c} from "c";
+    c++;
+    
+==== tests/cases/compiler/node_modules/b.ts (0 errors) ====
+    var a = 10;
+    
+==== tests/cases/compiler/node_modules/c.js (0 errors) ====
+    exports.a = 10;
+    c = 10;
+    

tests/cases/compiler/jsFileCompilationExternalPackageError.ts

@@ -0,0 +1,16 @@
+// @allowJs: true
+// @noEmit: true
+// @module: commonjs
+
+// @filename: moduleA/a.js
+import {a} from "b";
+a++;
+import {c} from "c";
+c++;
+
+// @filename: node_modules/b.ts
+var a = 10;
+
+// @filename: node_modules/c.js
+exports.a = 10;
+c = 10;