Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/elements/elements.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 elements;
 
 
 import '../constants/expressions.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
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   /// Trying to access a function signature that has not been computed in
   /// resolution is an error and calling [computeSignature] covers that error.
   /// This method will go away!
   // TODO(johnniwinther): Rename to `ensureFunctionSignature`.
   @deprecated FunctionSignature computeSignature(Compiler compiler);
 
   bool get hasFunctionSignature;
 
   /// The type of this function.
   FunctionType get type;
+
+  /// The synchronous/asynchronous marker on this function.
+  AsyncMarker get asyncMarker;
+}
+
+/// Enum for the synchronous/asynchronous function body modifiers.
+class AsyncMarker {
+  /// The default function body marker.
+  static AsyncMarker SYNC = const AsyncMarker._();
+
+  /// The `sync*` function body marker.
+  static AsyncMarker SYNC_STAR = const AsyncMarker._(isYielding: true);
+
+  /// The `async` function body marker.
+  static AsyncMarker ASYNC = const AsyncMarker._(isAsync: true);
+
+  /// The `async*` function body marker.
+  static AsyncMarker ASYNC_STAR =
+      const AsyncMarker._(isAsync: true, isYielding: true);
+
+  /// Is `true` if this marker defines the function body to have an
+  /// asynchronous result, that is, either a [Future] or a [Stream].
+  final bool isAsync;
+
+  /// Is `true` if this marker defines the function body to have a plural
+  /// result, that is, either an [Iterable] or a [Stream].
+  final bool isYielding;
+
+  const AsyncMarker._({this.isAsync: false, this.isYielding: false});
+
+  String toString() {
+    return '${isAsync ? 'async' : 'sync'}${isYielding ? '*' : ''}';
+  }
 }
 
 /// A top level, static or instance function.
 abstract class MethodElement extends FunctionElement
     implements MemberElement {
 }
 
 /// A local function or closure (anonymous local function).
 abstract class LocalFunctionElement extends FunctionElement
     implements LocalElement {
@@ skipping 419 matching lines @@
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }