fix(@angular-devkit/build-angular): close compiler on Karma exit

The webpack compiler was never closed which caused Webpack's shutdown hook https://github.com/angular/angular-cli/blob/c71832f3a9942aeb2c3dbd62f6edb4f0b8366465/packages/angular_devkit/build_angular/src/webpack/configs/styles.ts#L113-L114 to never be called and in turn caused the Sass workers never to be termined.

packages/angular_devkit/build_angular/src/webpack/plugins/karma/karma.ts

@@ -149,7 +149,7 @@ const init: any = (config: any, emitter: any) => {
   webpackMiddleware = webpackDevMiddleware(compiler, webpackMiddlewareConfig);
   emitter.on('exit', (done: any) => {
     webpackMiddleware.close();
-    done();
+    compiler.close(() => done());
   });
 
   function unblock() {