First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/mirrors_lookup_test.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.test.memory_source_file_helper;
 
 import 'package:expect/expect.dart';
+import "../../async_helper.dart";
 import 'memory_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors_util.dart';
 
 const Map MEMORY_SOURCE_FILES = const {
   'main.dart': r"""
 
 library main;
 
 import 'dart:async' as async show Future;
 
 var variable;
 
 method(a, [b]) {}
 
 class Class<A> {
   var field;
   var variable;
   method(c, {d}) {}
 }
 
 class Subclass<B> extends Class<B> {
   var subfield;
 }
 """,
 };
 
 void main() {
-  mirrorSystemFor(MEMORY_SOURCE_FILES).then(
+  asyncTest(() => mirrorSystemFor(MEMORY_SOURCE_FILES).then(
       (MirrorSystem mirrors) => test(mirrors),
-      onError: (e) => Expect.fail('$e'));
+      onError: (e) => Expect.fail('$e')));
 }
 
 void test(MirrorSystem mirrors) {
   LibraryMirror dartCore = mirrors.libraries[Uri.parse('dart:core')];
   Expect.isNotNull(dartCore);
 
   LibraryMirror dartAsync = mirrors.libraries[Uri.parse('dart:async')];
   Expect.isNotNull(dartAsync);
 
   LibraryMirror library = mirrors.libraries[Uri.parse('memory:main.dart')];
@@ skipping 203 matching lines @@
   Expect.equals(Subclass_subfield,
                 lookupQualifiedInScope(library, 'Subclass.subfield'));
 
   Expect.equals(Future_, lookupQualifiedInScope(library, 'async.Future'));
   Expect.isTrue(
       lookupQualifiedInScope(library, 'async.Future.then') is MethodMirror);
   // `Timer` should not be found through the prefix `async.Future`.
   Expect.isNull(
       lookupQualifiedInScope(library, 'async.Future.Timer'));
 }