dart2js cps: Support async/await by rewriting the JavaScript AST.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

 // Copyright (c) 2013, 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.ir_builder;
 
 import '../compile_time_constants.dart' show BackendConstantEnvironment;
 import '../constants/constant_system.dart';
 import '../constants/values.dart' show ConstantValue, PrimitiveConstantValue;
 import '../dart_types.dart';
@@ skipping 2687 matching lines @@
         sourceInformation: sourceInformation);
   }
 
   /// Convert the given value to a string.
   ir.Primitive buildStringify(ir.Primitive value) {
     return buildStaticFunctionInvocation(
         program.stringifyFunction,
         new CallStructure.unnamed(1),
         <ir.Primitive>[value]);
   }
+
+  ir.Node buildAwait(ir.Primitive value) {
+    return _continueWithExpression((k) => new ir.Await(value, k));
+  }
 }
 
 
 /// Location of a variable relative to a given closure.
 class ClosureLocation {
   /// If not `null`, this location is [box].[field].
   /// The location of [box] can be obtained separately from an
   /// enclosing [ClosureEnvironment] or [ClosureScope].
   /// If `null`, then the location is [field] on the enclosing function object.
   final BoxLocal box;
@@ skipping 61 matching lines @@
 }
 
 class SwitchCaseInfo {
   final List<ir.Primitive> constants = <ir.Primitive>[];
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildBody);
 
   void addConstant(ir.Primitive constant) => constants.add(constant);
 }