perf(@angular-devkit/build-angular): reduce babel transformation in esbuild builder

When using the experimental esbuild-based browser application builder, babel transformation
is now only performed on a file if the file requires the specific transformations enabled
for the build. This has the benefit of removing the need to parse and process files that
will not be affected by the enabled transformations.
From initial testing, this provides a 30% build time improvement for development builds of a
newly generated application and a 10% improvement for production builds.

Author: clydin

packages/angular_devkit/build_angular/src/builders/browser-esbuild/compiler-plugin.ts

@@ -323,11 +323,23 @@ export function createCompilerPlugin(
             };
           }
 
+          const data = typescriptResult.content ?? '';
+          const forceAsyncTransformation = /for\s+await\s*\(|async\s+function\s*\*/.test(data);
           const useInputSourcemap =
             pluginOptions.sourcemap &&
             (!!pluginOptions.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(args.path));
 
-          const data = typescriptResult.content ?? '';
+          // If no additional transformations are needed, return the TypeScript output directly
+          if (!forceAsyncTransformation && !pluginOptions.advancedOptimizations) {
+            return {
+              // Strip sourcemaps if they should not be used
+              contents: useInputSourcemap
+                ? data
+                : data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, ''),
+              loader: 'js',
+            };
+          }
+
           const babelResult = await transformAsync(data, {
             filename: args.path,
             inputSourceMap: (useInputSourcemap ? undefined : false) as undefined,
@@ -341,7 +353,7 @@ export function createCompilerPlugin(
               [
                 angularApplicationPreset,
                 {
-                  forceAsyncTransformation: /for\s+await\s*\(|async\s+function\s*\*/.test(data),
+                  forceAsyncTransformation,
                   optimize: pluginOptions.advancedOptimizations && {},
                 },
               ],
@@ -356,6 +368,26 @@ export function createCompilerPlugin(
       );
 
       build.onLoad({ filter: /\.[cm]?js$/ }, async (args) => {
+        const data = await fs.readFile(args.path, 'utf-8');
+        const forceAsyncTransformation =
+          !/[\\/][_f]?esm2015[\\/]/.test(args.path) &&
+          /for\s+await\s*\(|async\s+function\s*\*/.test(data);
+        const shouldLink = await requiresLinking(args.path, data);
+        const useInputSourcemap =
+          pluginOptions.sourcemap &&
+          (!!pluginOptions.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(args.path));
+
+        // If no additional transformations are needed, return the TypeScript output directly
+        if (!forceAsyncTransformation && !pluginOptions.advancedOptimizations && !shouldLink) {
+          return {
+            // Strip sourcemaps if they should not be used
+            contents: useInputSourcemap
+              ? data
+              : data.replace(/^\/\/# sourceMappingURL=[^\r\n]*/gm, ''),
+            loader: 'js',
+          };
+        }
+
         const angularPackage = /[\\/]node_modules[\\/]@angular[\\/]/.test(args.path);
 
         const linkerPluginCreator = (
@@ -364,11 +396,6 @@ export function createCompilerPlugin(
           )
         ).createEs2015LinkerPlugin;
 
-        const useInputSourcemap =
-          pluginOptions.sourcemap &&
-          (!!pluginOptions.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(args.path));
-
-        const data = await fs.readFile(args.path, 'utf-8');
         const result = await transformAsync(data, {
           filename: args.path,
           inputSourceMap: (useInputSourcemap ? undefined : false) as undefined,
@@ -383,13 +410,11 @@ export function createCompilerPlugin(
               angularApplicationPreset,
               {
                 angularLinker: {
-                  shouldLink: await requiresLinking(args.path, data),
+                  shouldLink,
                   jitMode: false,
                   linkerPluginCreator,
                 },
-                forceAsyncTransformation:
-                  !/[\\/][_f]?esm2015[\\/]/.test(args.path) &&
-                  /for\s+await\s*\(|async\s+function\s*\*/.test(data),
+                forceAsyncTransformation,
                 optimize: pluginOptions.advancedOptimizations && {
                   looseEnums: angularPackage,
                   pureTopLevel: angularPackage,