refactor(@angular-devkit/build-angular): remove legacy ES5 build optimizer usage

The legacy standalone build optimizer was still used when targetting ES5 due to the new babel-based build optimizer only supporting the analysis of ES2015+ code. Mainly due to the presence of native classes, the new build optimizer is faster and significantly less complex. To fully remove the legacy build optimizer, ES5 downleveling of application code is now also performed by babel when the TypeScript configuration specifies an ES5 target. TypeScript will now internally emit ES2015 in this configuration and ES2015 code will be used as the input to the remainder of the build pipeline. This has been the behavior for third-party packages and now all code will be consistently downleveled to ES5 using the same tooling.

Author: clydin

packages/angular_devkit/build_angular/BUILD.bazel

@@ -94,7 +94,6 @@ ts_library(
     module_root = "src/index.d.ts",
     deps = [
         "//packages/angular_devkit/architect",
-        "//packages/angular_devkit/build_optimizer",
         "//packages/angular_devkit/build_webpack",
         "//packages/angular_devkit/core",
         "//packages/angular_devkit/core/node",

packages/angular_devkit/build_angular/package.json

@@ -8,7 +8,6 @@
   "dependencies": {
     "@ampproject/remapping": "1.0.1",
     "@angular-devkit/architect": "0.0.0",
-    "@angular-devkit/build-optimizer": "0.0.0",
     "@angular-devkit/build-webpack": "0.0.0",
     "@angular-devkit/core": "0.0.0",
     "@babel/core": "7.15.0",

packages/angular_devkit/build_angular/src/babel/webpack-loader.ts

@@ -66,8 +66,7 @@ export default custom<AngularCustomOptions>(() => {
       const esTarget = scriptTarget as ScriptTarget | undefined;
       if (esTarget !== undefined) {
         if (esTarget < ScriptTarget.ES2015) {
-          // TypeScript files will have already been downlevelled
-          customOptions.forceES5 = !/\.tsx?$/.test(this.resourcePath);
+          customOptions.forceES5 = true;
         } else if (esTarget >= ScriptTarget.ES2017) {
           customOptions.forceAsyncTransformation =
             !/[\\\/][_f]?esm2015[\\\/]/.test(this.resourcePath) && source.includes('async');

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

@@ -6,10 +6,6 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {
-  BuildOptimizerWebpackPlugin,
-  buildOptimizerLoaderPath,
-} from '@angular-devkit/build-optimizer';
 import {
   GLOBAL_DEFS_FOR_TERSER,
   GLOBAL_DEFS_FOR_TERSER_WITH_AOT,
@@ -301,17 +297,6 @@ export function getCommonConfig(wco: WebpackConfigOptions): Configuration {
     });
   }
 
-  let buildOptimizerUseRule: RuleSetRule[] = [];
-  if (buildOptions.buildOptimizer && wco.scriptTarget < ScriptTarget.ES2015) {
-    extraPlugins.push(new BuildOptimizerWebpackPlugin());
-    buildOptimizerUseRule = [
-      {
-        loader: buildOptimizerLoaderPath,
-        options: { sourceMap: scriptsSourceMap },
-      },
-    ];
-  }
-
   const extraMinimizers = [];
 
   if (scriptsOptimization) {
@@ -422,10 +407,9 @@ export function getCommonConfig(wco: WebpackConfigOptions): Configuration {
                 cacheDirectory: findCachePath('babel-webpack'),
                 scriptTarget: wco.scriptTarget,
                 aot: buildOptions.aot,
-                optimize: buildOptions.buildOptimizer && wco.scriptTarget >= ScriptTarget.ES2015,
+                optimize: buildOptions.buildOptimizer,
               },
             },
-            ...buildOptimizerUseRule,
           ],
         },
         ...extraRules,

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

@@ -9,6 +9,7 @@
 import { getSystemPath } from '@angular-devkit/core';
 import { CompilerOptions } from '@angular/compiler-cli';
 import { AngularWebpackLoaderPath, AngularWebpackPlugin } from '@ngtools/webpack';
+import { ScriptTarget } from 'typescript';
 import { Configuration } from 'webpack';
 import { WebpackConfigOptions } from '../../utils/build-options';
 
@@ -45,6 +46,13 @@ function createIvyPlugin(
     compilerOptions.preserveSymlinks = buildOptions.preserveSymlinks;
   }
 
+  // Outputting ES2015 from TypeScript is the required minimum for the build optimizer passes.
+  // Downleveling to ES5 will occur after the build optimizer passes via babel which is the same
+  // as for third-party libraries. This greatly reduces the complexity of static analysis.
+  if (wco.scriptTarget < ScriptTarget.ES2015) {
+    compilerOptions.target = ScriptTarget.ES2015;
+  }
+
   const fileReplacements: Record<string, string> = {};
   if (buildOptions.fileReplacements) {
     for (const replacement of buildOptions.fileReplacements) {