Make support for async/await syntax optional in the frontend.

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.compiler_base;
 
 import 'dart:async' show EventSink, Future;
 
 import '../compiler_new.dart' as api;
 import 'cache_strategy.dart' show CacheStrategy;
@@ skipping 419 matching lines @@
   /// been scanned.
   ///
   /// Use this callback method to store references to specific member declared
   /// in certain libraries. Note that [library] has not been patched yet, nor
   /// has its imports/exports been resolved.
   ///
   /// Use [loader] to register the creation and scanning of a patch library
   /// for [library].
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader) {
     Uri uri = library.canonicalUri;
-    if (uri == Uris.dart_core) {
-      initializeCoreClasses();
-      identicalFunction = coreLibrary.find('identical');
-    } else if (uri == Uris.dart_mirrors) {
-      mirrorSystemClass = findRequiredElement(library, 'MirrorSystem');
-      mirrorsUsedClass = findRequiredElement(library, 'MirrorsUsed');
-    } else if (uri == Uris.dart_async) {
-      asyncLibrary = library;
-      deferredLibraryClass = findRequiredElement(library, 'DeferredLibrary');
-      _coreTypes.futureClass = findRequiredElement(library, 'Future');
-      _coreTypes.streamClass = findRequiredElement(library, 'Stream');
-    } else if (uri == Uris.dart__native_typed_data) {
-      typedDataClass = findRequiredElement(library, 'NativeTypedData');
-    } else if (uri == js_backend.BackendHelpers.DART_JS_HELPER) {
-      patchAnnotationClass = findRequiredElement(library, '_Patch');
-      nativeAnnotationClass = findRequiredElement(library, 'Native');
+    // If the script of the library is synthesized, the library does not exist
+    // and we do not try to load the helpers.
+    //
+    // This could for example happen if dart:async is disabled, then loading it
+    // should not try to find the given element.
+    // TODO(johnniwinther): This should just be library.isSynthesized, but that
+    // does not work yet for deserialized elements.
+    if (!library.compilationUnit.script.isSynthesized) {
+      if (uri == Uris.dart_core) {
+        initializeCoreClasses();
+        identicalFunction = coreLibrary.find('identical');
+      } else if (uri == Uris.dart_mirrors) {
+        mirrorSystemClass = findRequiredElement(library, 'MirrorSystem');
+        mirrorsUsedClass = findRequiredElement(library, 'MirrorsUsed');
+      } else if (uri == Uris.dart_async) {
+        asyncLibrary = library;
+        deferredLibraryClass = findRequiredElement(library, 'DeferredLibrary');
+        _coreTypes.futureClass = findRequiredElement(library, 'Future');
+        _coreTypes.streamClass = findRequiredElement(library, 'Stream');
+      } else if (uri == Uris.dart__native_typed_data) {
+        typedDataClass = findRequiredElement(library, 'NativeTypedData');
+      } else if (uri == js_backend.BackendHelpers.DART_JS_HELPER) {
+        patchAnnotationClass = findRequiredElement(library, '_Patch');
+        nativeAnnotationClass = findRequiredElement(library, 'Native');
+      }
     }
     return backend.onLibraryScanned(library, loader);
   }
 
   /// Compute the set of distinct import chains to the library at [uri] within
   /// [loadedLibraries].
   ///
   /// The chains are strings of the form
   ///
   ///       <main-uri> => <intermediate-uri1> => <intermediate-uri2> => <uri>
@@ skipping 1595 matching lines @@
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
 
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
   String valueOf(String key) => null;
 }