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

pkg/compiler/lib/src/js_backend/backend.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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 209 matching lines @@
   EMPTY_VALUE,
   TYPEVARIABLE_REFERENCE, // Reference to a type in reflection data.
   NAME
 }
 
 class JavaScriptBackend extends Backend {
   String get patchVersion => emitter.patchVersion;
 
   bool get supportsReflection => emitter.emitter.supportsReflection;
 
+  bool get supportsAsyncAwait => true;
+
   final Annotations annotations;
 
   /// Set of classes that need to be considered for reflection although not
   /// otherwise visible during resolution.
   Iterable<ClassElement> get classesRequiredForReflection {
     // TODO(herhut): Clean this up when classes needed for rti are tracked.
     return [helpers.closureClass, helpers.jsIndexableClass];
   }
 
   FunctionCompiler functionCompiler;
@@ skipping 2776 matching lines @@
 
   @override
   void onImpactUsed(ImpactUseCase impactUse) {
     if (impactUse == DeferredLoadTask.IMPACT_USE && !supportSerialization) {
       // TODO(johnniwinther): Allow emptying when serialization has been
       // performed.
       resolution.emptyCache();
     }
   }
 }