fix(@ngtools/webpack): change of CSS no longer breaks rebuild

Fixes #4326
Fixes #4329

Author: Brocco

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

@@ -187,8 +187,10 @@ export class WebpackCompilerHost implements ts.CompilerHost {
   }
 
   invalidate(fileName: string): void {
-    this._files[fileName] = null;
-    this._changedFiles[fileName] = true;
+    if (fileName in this._files) {
+      this._files[fileName] = null;
+      this._changedFiles[fileName] = true;
+    }
   }
 
   fileExists(fileName: string): boolean {

tests/e2e/tests/build/rebuild-css-change.ts

@@ -0,0 +1,27 @@
+import {
+  killAllProcesses,
+  exec,
+  waitForAnyProcessOutputToMatch,
+  silentExecAndWaitForOutputToMatch
+} from '../../utils/process';
+import {appendToFile} from '../../utils/fs';
+
+
+export default function() {
+  if (process.platform.startsWith('win')) {
+    return Promise.resolve();
+  }
+
+  return silentExecAndWaitForOutputToMatch('ng', ['serve'], /webpack: bundle is now VALID|webpack: Compiled successfully./)
+    // Should trigger a rebuild.
+    .then(() => exec('touch', 'src/main.ts'))
+    .then(() => waitForAnyProcessOutputToMatch(/webpack: bundle is now INVALID|webpack: Compiling.../, 1000))
+    .then(() => waitForAnyProcessOutputToMatch(/webpack: bundle is now VALID|webpack: Compiled successfully./, 5000))
+    .then(() => appendToFile('src/app/app.component.css', ':host { color: blue; }'))
+    .then(() => waitForAnyProcessOutputToMatch(/webpack: bundle is now INVALID|webpack: Compiling.../, 1000))
+    .then(() => waitForAnyProcessOutputToMatch(/webpack: bundle is now VALID|webpack: Compiled successfully./, 5000))
+    .then(() => killAllProcesses(), (err: any) => {
+      killAllProcesses();
+      throw err;
+    });
+}