Fix error handling in dart2js async-await

sdk/lib/_internal/compiler/js_lib/shared/async_await_error_codes.dart

Index: sdk/lib/_internal/compiler/js_lib/shared/async_await_error_codes.dart
diff --git a/pkg/compiler/lib/src/js/js_ast.dart b/sdk/lib/_internal/compiler/js_lib/shared/async_await_error_codes.dart
similarity index 53%
copy from pkg/compiler/lib/src/js/js_ast.dart
copy to sdk/lib/_internal/compiler/js_lib/shared/async_await_error_codes.dart
index ee8289ecf2df0aa8bf0c73727d8dac478d223ff5..f87406b31b03e160c74776d5f043578f55603767 100644
--- a/pkg/compiler/lib/src/js/js_ast.dart
+++ b/sdk/lib/_internal/compiler/js_lib/shared/async_await_error_codes.dart
@@ -2,12 +2,9 @@
 // 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 js_ast;
+/// Contains error codes that transformed async/async* functions use to
+/// communicate with js_helper functions.
 
-import 'precedence.dart';
-import 'characters.dart' as charCodes;
-
-part 'nodes.dart';
-part 'builder.dart';
-part 'printer.dart';
-part 'template.dart';
+const int SUCCESS = 0;
+const int ERROR = 1;
+const int STREAM_WAS_CANCELED = 2;