First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/no_duplicate_constructor_body2_test.dart

 // Copyright (c) 2012, 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.
 
 import 'compiler_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 const String CODE = """
 var x = 0;
 class A {
   A() { x++; }
 }
 
 class B extends A {
   B();
 }
 
 main() {
   new B();
   new A();
 }
 """;
 
 main() {
-  String generated = compileAll(CODE);
-  RegExp regexp = new RegExp(r'A\$0: function');
-  Iterator<Match> matches = regexp.allMatches(generated).iterator;
-  checkNumberOfMatches(matches, 1);
+  asyncTest(() => compileAll(CODE).then((generated) {
+    RegExp regexp = new RegExp(r'A\$0: function');
+    Iterator<Match> matches = regexp.allMatches(generated).iterator;
+    checkNumberOfMatches(matches, 1);
+  }));
 }