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

pkg/compiler/lib/src/common/backend_api.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.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 37 matching lines @@
     ItemCompilationContext;
 
 abstract class Backend {
   final Compiler compiler;
 
   Backend(this.compiler);
 
   /// Returns true if the backend supports reflection.
   bool get supportsReflection;
 
+  /// Returns true if the backend supports reflection.
+  bool get supportsAsyncAwait;
+
   /// The [ConstantSystem] used to interpret compile-time constants for this
   /// backend.
   ConstantSystem get constantSystem;
 
   /// The constant environment for the backend interpretation of compile-time
   /// constants.
   BackendConstantEnvironment get constants;
 
   /// The compiler task responsible for the compilation of constants for both
   /// the frontend and the backend.
@@ skipping 366 matching lines @@
   }
 }
 
 /// Interface for serialization of backend specific data.
 class BackendSerialization {
   const BackendSerialization();
 
   SerializerPlugin get serializer => const SerializerPlugin();
   DeserializerPlugin get deserializer => const DeserializerPlugin();
 }