First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/string_interpolation_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';
 
 main() {
-  String code =
-      compileAll(r'''main() { return "${2}${true}${'a'}${3.14}"; }''');
-  Expect.isTrue(code.contains(r'2truea3.14'));
+  compileAll(r'''main() { return "${2}${true}${'a'}${3.14}"; }''').then((code) {
+    Expect.isTrue(code.contains(r'2truea3.14'));
+  });
 
-  code = compileAll(r'''main() { return "foo ${new Object()}"; }''');
-  Expect.isFalse(code.contains(r'concat'));
+  compileAll(r'''main() { return "foo ${new Object()}"; }''').then((code) {
+    Expect.isFalse(code.contains(r'concat'));
+  });
 }