Fix error handling in dart2js async-await

tests/compiler/dart2js/mock_libraries.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 for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 80 matching lines @@
 import 'dart:_interceptors';
 import 'dart:_isolate_helper';
 import 'dart:async';
 ''';
 
 const Map<String, String> DEFAULT_JS_HELPER_LIBRARY = const <String, String>{
   'assertHelper': 'assertHelper(a) {}',
   'assertIsSubtype': 'assertIsSubtype(subtype, supertype, message) {}',
   'assertSubtype': 'assertSubtype(object, isField, checks, asField) {}',
   'assertSubtypeOfRuntimeType': 'assertSubtypeOfRuntimeType(object, type) {}',
+  'asyncHelper': 'asyncHelper(object, asyncBody, completer) {}',
   'boolConversionCheck': 'boolConversionCheck(x) {}',
   'boolTypeCast': 'boolTypeCast(value) {}',
   'boolTypeCheck': 'boolTypeCheck(value) {}',
   'checkSubtype': 'checkSubtype(object, isField, checks, asField) {}',
   'checkSubtypeOfRuntimeType': 'checkSubtypeOfRuntimeType(o, t) {}',
   'BoundClosure': r'''abstract class BoundClosure extends Closure {
     var self;
     var target;
     var receiver;
   }''',
@@ skipping 91 matching lines @@
   'S': 'S() {}',
   'setRuntimeTypeInfo': 'setRuntimeTypeInfo(a, b) {}',
   'stringSuperNativeTypeCast': 'stringSuperNativeTypeCast(value) {}',
   'stringSuperNativeTypeCheck': 'stringSuperNativeTypeCheck(value) {}',
   'stringSuperTypeCast': 'stringSuperTypeCast(value) {}',
   'stringSuperTypeCheck': 'stringSuperTypeCheck(value) {}',
   'stringTypeCast': 'stringTypeCast(x) {}',
   'stringTypeCheck': 'stringTypeCheck(x) {}',
   'subtypeCast': 'subtypeCast(object, isField, checks, asField) {}',
   'subtypeOfRuntimeTypeCast': 'subtypeOfRuntimeTypeCast(object, type) {}',
-  'thenHelper':
-      'thenHelper(object, helperCallback, completer, errorCallback) {}',
   'throwAbstractClassInstantiationError':
       'throwAbstractClassInstantiationError(className) {}',
   'throwCyclicInit': 'throwCyclicInit(staticName) {}',
   'throwExpression': 'throwExpression(e) {}',
   'throwNoSuchMethod':
       'throwNoSuchMethod(obj, name, arguments, expectedArgumentNames) {}',
   'throwRuntimeError': 'throwRuntimeError(message) {}',
   'throwTypeError': 'throwTypeError(message) {}',
   'TypeImpl': 'class TypeImpl {}',
   'TypeVariable': 'class TypeVariable {}',
@@ skipping 146 matching lines @@
   'Stream': 'class Stream<T> {}',
   'Completer': 'class Completer<T> {}',
   'StreamIterator': 'class StreamIterator<T> {}',
 };
 
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };