Support async/await in the dart2js frontend.

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index 5f5cc744ea8ac2f17dd80265f4d92b7f8c692e20..f0d2b720d950983dc5e3d03b41ea6dac5d27b82f 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -42,6 +42,7 @@ const Map<String, String> DEFAULT_CORE_LIBRARY = const <String, String>{
   'Function': 'class Function {}',
   'identical': 'bool identical(Object a, Object b) { return true; }',
   'int': 'abstract class int extends num { }',
+  'Iterable': 'abstract class Iterable {}',
   'LinkedHashMap': r'''
       class LinkedHashMap {
         factory LinkedHashMap._empty() => null;
@@ -357,6 +358,11 @@ const Map<String, String> DEFAULT_ISOLATE_HELPER_LIBRARY =
   '_WorkerBase': 'class _WorkerBase {}',
 };
 
+const Map<String, String> DEFAULT_ASYNC_LIBRARY = const <String, String>{
+  'Future': 'class Future<T> {}',
+  'Stream': 'class Stream<T> {}',
+};
+
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',