refactor(@angular-devkit/build-angular): use built-in async generator downleveling in esbuild

As of esbuild v0.18.8, async generators can be downleveled when not supported. This improvement
allows for the removal of the babel plugins to perform the async generator downleveling that were
previously used. This can provide a performance benefit for cases of async generator usage in
application and third-party library code.

Author: clydin

packages/angular_devkit/build_angular/src/tools/esbuild/javascript-transformer-worker.ts

@@ -18,7 +18,6 @@ interface JavaScriptTransformRequest {
   sourcemap: boolean;
   thirdPartySourcemaps: boolean;
   advancedOptimizations: boolean;
-  forceAsyncTransformation?: boolean;
   skipLinker: boolean;
   jit: boolean;
 }
@@ -41,16 +40,13 @@ async function transformWithBabel({
   data,
   ...options
 }: JavaScriptTransformRequest): Promise<string> {
-  const forceAsyncTransformation =
-    options.forceAsyncTransformation ??
-    (!/[\\/][_f]?esm2015[\\/]/.test(filename) && /async(?:\s+function)?\s*\*/.test(data));
   const shouldLink = !options.skipLinker && (await requiresLinking(filename, data));
   const useInputSourcemap =
     options.sourcemap &&
     (!!options.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(filename));
 
   // If no additional transformations are needed, return the data directly
-  if (!forceAsyncTransformation && !options.advancedOptimizations && !shouldLink) {
+  if (!options.advancedOptimizations && !shouldLink) {
     // Strip sourcemaps if they should not be used
     return useInputSourcemap ? data : data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, '');
   }
@@ -87,7 +83,6 @@ async function transformWithBabel({
             jitMode: options.jit,
             linkerPluginCreator,
           },
-          forceAsyncTransformation,
           optimize: options.advancedOptimizations && {
             pureTopLevel: safeAngularPackage,
           },

packages/angular_devkit/build_angular/src/tools/esbuild/javascript-transformer.ts

@@ -58,7 +58,7 @@ export class JavaScriptTransformer {
    * @returns A promise that resolves to a UTF-8 encoded Uint8Array containing the result.
    */
   transformFile(filename: string, skipLinker?: boolean): Promise<Uint8Array> {
-    // Always send the request to a worker. Files are almost always from node modules which measn
+    // Always send the request to a worker. Files are almost always from node modules which means
     // they may need linking. The data is also not yet available to perform most transformation checks.
     return this.#workerPool.run({
       filename,
@@ -78,29 +78,20 @@ export class JavaScriptTransformer {
   async transformData(filename: string, data: string, skipLinker: boolean): Promise<Uint8Array> {
     // Perform a quick test to determine if the data needs any transformations.
     // This allows directly returning the data without the worker communication overhead.
-    let forceAsyncTransformation;
     if (skipLinker && !this.#commonOptions.advancedOptimizations) {
-      // If the linker is being skipped and no optimizations are needed, only async transformation is left.
-      // This checks for async generator functions and class methods. All other async transformation is handled by esbuild.
-      forceAsyncTransformation = data.includes('async') && /async(?:\s+function)?\s*\*/.test(data);
+      const keepSourcemap =
+        this.#commonOptions.sourcemap &&
+        (!!this.#commonOptions.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(filename));
 
-      if (!forceAsyncTransformation) {
-        const keepSourcemap =
-          this.#commonOptions.sourcemap &&
-          (!!this.#commonOptions.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(filename));
-
-        return Buffer.from(
-          keepSourcemap ? data : data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, ''),
-          'utf-8',
-        );
-      }
+      return Buffer.from(
+        keepSourcemap ? data : data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, ''),
+        'utf-8',
+      );
     }
 
     return this.#workerPool.run({
       filename,
       data,
-      // Send the async check result if present to avoid rechecking in the worker
-      forceAsyncTransformation,
       skipLinker,
       ...this.#commonOptions,
     });

packages/angular_devkit/build_angular/src/tools/esbuild/utils.ts

@@ -123,10 +123,7 @@ export async function logMessages(
 export function getFeatureSupport(target: string[]): BuildOptions['supported'] {
   const supported: Record<string, boolean> = {
     // Native async/await is not supported with Zone.js. Disabling support here will cause
-    // esbuild to downlevel async/await and for await...of to a Zone.js supported form. However, esbuild
-    // does not currently support downleveling async generators. Instead babel is used within the JS/TS
-    // loader to perform the downlevel transformation.
-    // NOTE: If esbuild adds support in the future, the babel support for async generators can be disabled.
+    // esbuild to downlevel async/await, async generators, and for await...of to a Zone.js supported form.
     'async-await': false,
     // V8 currently has a performance defect involving object spread operations that can cause signficant
     // degradation in runtime performance. By not supporting the language feature here, a downlevel form