First pass at asynchronous input loading in dart2js.

tests/compiler/dart2js/regress_10231_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.
 
 // Regression test for http://dartbug.com/10231.
 
 import 'package:expect/expect.dart';
 import 'codegen_helper.dart';
 
 void main() {
-  var code = generate(SOURCE)['test'];
-  Expect.isNotNull(code);
-  Expect.equals(0, new RegExp('add').allMatches(code).length);
-  Expect.equals(3, new RegExp('\\+').allMatches(code).length);
+  generate(SOURCE).then((result) {
+    var code = result['test'];
+    Expect.isNotNull(code);
+    Expect.equals(0, new RegExp('add').allMatches(code).length);
+    Expect.equals(3, new RegExp('\\+').allMatches(code).length);
+  });
 }
 
 const String SOURCE = """
 test(a, b, c, d) {
   if (a is !num) throw 'a not num';
   if (b is !num) throw 'b not num';
   if (c is !num) throw 'c not num';
   if (d is !num) throw 'd not num';
   return a + b + c + d;
 }
 
 main() {
   test(1, 2, 3, 4);
   test('x', 'y', 'z', 'w');
   test([], {}, [], {});
 }
 """;