fix(@ngtools/webpack): do not remove outDir from the configuration

It would overwrite JavaScript files in the source directory. Even if its a virtual
compiler host, this is still a BAD THING and TypeScript complains.

Fixes angular/angular#21080
Might help with #8371
Fixes #8885

packages/@ngtools/webpack/src/angular_compiler_plugin.ts

@@ -177,9 +177,6 @@ export class AngularCompilerPlugin implements Tapable {
     this._compilerOptions = { ...config.options, ...options.compilerOptions };
     this._basePath = config.options.basePath;
 
-    // Overwrite outDir so we can find generated files next to their .ts origin in compilerHost.
-    this._compilerOptions.outDir = '';
-
     // Default plugin sourceMap to compiler options setting.
     if (!options.hasOwnProperty('sourceMap')) {
       options.sourceMap = this._compilerOptions.sourceMap || false;
@@ -793,8 +790,21 @@ export class AngularCompilerPlugin implements Tapable {
     }
   }
 
+  private _getOutputPathFromSourcePath(fileName: string) {
+    if (fileName.startsWith(this._basePath)) {
+      return fileName
+        .substr(this._basePath.length)
+        .replace(/^/, this._compilerOptions.outDir + '/')
+        .replace(/.ts$/, '.js');
+    } else if (path.isAbsolute(fileName)) {
+      return fileName;
+    } else {
+      return path.join(this._compilerOptions.outDir);
+    }
+  }
+
   getCompiledFile(fileName: string) {
-    const outputFile = fileName.replace(/.ts$/, '.js');
+    const outputFile = this._getOutputPathFromSourcePath(fileName);
     let outputText: string;
     let sourceMap: string;
     let errorDependencies: string[] = [];

tests/e2e/tests/build/allow-js.ts

@@ -0,0 +1,18 @@
+import { ng } from '../../utils/process';
+import { copyProjectAsset } from '../../utils/assets';
+import { expectFileToMatch, writeMultipleFiles } from '../../utils/fs';
+import { updateJsonFile } from '../../utils/project';
+
+
+export default function () {
+  return Promise.resolve()
+    .then(() => writeMultipleFiles({
+      // Doesn't need to have content or be imported from anywhere.
+      'src/some-javascript.js': '',
+    }))
+    .then(() => updateJsonFile('tsconfig.json', json => {
+      json['compilerOptions']['allowJs'] = true;
+    }))
+    // We only test that the build passes.
+    .then(() => ng('build'))
+}