First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/no_constructor_body_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 "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const String TEST = r"""
 class A {
   A.foo() {}
   A();
 }
 main() {
   new A();
   new A.foo();
 }
 """;
 
 main() {
-  String generated = compileAll(TEST);
-  Expect.isTrue(
-      generated.contains('A: {"": "Object;"}'));
+  compileAll(TEST).then((generated) {
+    Expect.isTrue(generated.contains('A: {"": "Object;"}'));
+  });
 }