refactor(@angular-devkit/build-angular): enable babel build optimizer passes for ES2015+ targets

The new babel-based build optimizer passes are now enabled when targetting ES2015 and higher. ES5 targets will currently continue to use the previous build optimizer. ES5 support will require additional refactoring of the downlevelling process as the new build optimizer passes are designed for ES2015 code.

Author: clydin

packages/angular_devkit/build_angular/src/webpack/configs/common.ts

@@ -296,7 +296,7 @@ export function getCommonConfig(wco: WebpackConfigOptions): Configuration {
   }
 
   let buildOptimizerUseRule: RuleSetRule[] = [];
-  if (buildOptions.buildOptimizer) {
+  if (buildOptions.buildOptimizer && wco.scriptTarget < ScriptTarget.ES2015) {
     extraPlugins.push(new BuildOptimizerWebpackPlugin());
     buildOptimizerUseRule = [
       {
@@ -460,6 +460,7 @@ export function getCommonConfig(wco: WebpackConfigOptions): Configuration {
                 cacheDirectory: findCachePath('babel-webpack'),
                 scriptTarget: wco.scriptTarget,
                 aot: buildOptions.aot,
+                optimize: buildOptions.buildOptimizer && wco.scriptTarget >= ScriptTarget.ES2015,
               },
             },
             ...buildOptimizerUseRule,