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

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

 // Copyright (c) 2014, 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 code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../tree_ir/tree_ir_nodes.dart' show BuiltinOperator;
@@ skipping 809 matching lines @@
         return js.js("typeof # === 'number'", args);
       case BuiltinOperator.IsNotNumber:
         return js.js("typeof # !== 'number'", args);
       case BuiltinOperator.IsFloor:
         return js.js("Math.floor(#) === #", args);
       case BuiltinOperator.IsNumberAndFloor:
         return js.js("typeof # === 'number' && Math.floor(#) === #", args);
     }
   }
 
+  @override
+  js.Expression visitAwait(tree_ir.Await node) {
+    return new js.Await(visitExpression(node.input));
+  }
+
   visitFunctionExpression(tree_ir.FunctionExpression node) {
     // FunctionExpressions are currently unused.
     // We might need them if we want to emit raw JS nested functions.
     throw 'FunctionExpressions should not be used';
   }
 }