perf(@angular-devkit/build-angular): remove Webpack Stats.toJson usage in karma plugin

Webpack's `Stats.toJson` function is an expensive operation and is recommended to be avoided where possible. In the case of the karma plugin, the compilation errors can be accessed directly without the need for the function call.

(cherry picked from commit 699b641)

Author: clydin

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

@@ -168,7 +168,7 @@ const init: any = (config: any, emitter: any) => {
   webpackConfig.output.path = `/${KARMA_APPLICATION_PATH}/`;
   webpackConfig.output.publicPath = `/${KARMA_APPLICATION_PATH}/`;
 
-  let compiler: any;
+  let compiler;
   try {
     compiler = webpack(webpackConfig);
   } catch (e) {
@@ -201,11 +201,10 @@ const init: any = (config: any, emitter: any) => {
 
   let lastCompilationHash: string | undefined;
   const statsConfig = getWebpackStatsConfig();
-  compiler.hooks.done.tap('karma', (stats: any) => {
-    if (stats.compilation.errors.length > 0) {
-      const json = stats.toJson(config.stats);
+  compiler.hooks.done.tap('karma', (stats) => {
+    if (stats.hasErrors()) {
       // Print compilation errors.
-      logger.error(statsErrorsToString(json, statsConfig));
+      logger.error(statsErrorsToString(stats.compilation, statsConfig));
       lastCompilationHash = undefined;
       // Emit a failure build event if there are compilation errors.
       failureCb();