First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/dead_code_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.
 
 import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 
 String TEST = r'''
 main() {
   foo(null);
 }
 foo(a) {
   if (a != null) return 42;
   return 54;
 }
 ''';
 
 main() {
-  String generated = compileAll(TEST);
-  Expect.isFalse(generated.contains('return 42'), 'dead code not eliminated');
+  compileAll(TEST).then((generated) {
+    Expect.isFalse(generated.contains('return 42'), 'dead code not eliminated');
+  });
 }