First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/emit_const_fields_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.
 // Test that unused static consts are not emitted.
 
 import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 const String TEST_GUIDE = r"""
 class Guide {
   static const LTUAE = 42;
   static const TITLE = 'Life, the Universe and Everything';
 }
 
 main() {
   return "${Guide.LTUAE}, ${Guide.TITLE}";
 }
 """;
 
 main() {
-  String generated = compileAll(TEST_GUIDE);
-  Expect.isTrue(generated.contains("42"));
-  Expect.isFalse(generated.contains("TITLE"));
+  compileAll(TEST_GUIDE).then((generated) {
+    Expect.isTrue(generated.contains("42"));
+    Expect.isFalse(generated.contains("TITLE"));
+  });
 }