fix(@angular-devkit/build-angular): remove certain redundant build steps from second build in differential loading

With this change we remove styles compilation and copying of assets for the second build when differential loading is enabled.

This should improve the build times for larger applications.

Author: alan-agius4

packages/angular_devkit/build_angular/src/utils/webpack-browser-config.ts

@@ -68,20 +68,29 @@ export async function generateWebpackConfig(
   // For differential loading, we can have several targets
   return scriptTargets.map(scriptTarget => {
     let buildOptions: NormalizedBrowserBuilderSchema = { ...options };
+    const supportES2015
+      = scriptTarget !== ts.ScriptTarget.ES3 && scriptTarget !== ts.ScriptTarget.ES5;
+
     if (differentialLoading) {
-      // For differential loading, the builder needs to created the index.html by itself
-      // without using a webpack plugin.
       buildOptions = {
         ...options,
+        ...(
+          // FIXME: we do create better webpack config composition to achieve the below
+          // When DL is enabled and supportES2015 is true it means that we are on the second build
+          // This also means that we don't need to include styles and assets multiple times
+          supportES2015
+            ? {}
+            : {
+              styles: options.extractCss ? [] : options.styles,
+              assets: [],
+            }
+        ),
         es5BrowserSupport: undefined,
         esVersionInFileName: true,
         scriptTargetOverride: scriptTarget,
       };
     }
 
-    const supportES2015
-      = scriptTarget !== ts.ScriptTarget.ES3 && scriptTarget !== ts.ScriptTarget.ES5;
-
     const wco: BrowserWebpackConfigOptions = {
       root: workspaceRoot,
       logger: logger.createChild('webpackConfigOptions'),