First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/field_codegen_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.
 // Test that parameters keep their names in the output.
 
 import "package:expect/expect.dart";
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
 
 const String TEST_NULL0 = r"""
 class A { static var x; }
 
 main() { return A.x; }
 """;
 
 const String TEST_NULL1 = r"""
 var x;
 
 main() { return x; }
 """;
 
 main() {
-  String generated = compileAll(TEST_NULL0);
-  Expect.isTrue(generated.contains("null"));
-  generated = compileAll(TEST_NULL1);
-  Expect.isTrue(generated.contains("null"));
+  compileAll(TEST_NULL0).then((generated) {
+    Expect.isTrue(generated.contains("null"));
+  });
+
+  compileAll(TEST_NULL1).then((generated) {
+    Expect.isTrue(generated.contains("null"));
+  });
 }