Support for a dart2js `dartDeferredLoader` hook.

tests/language/deferred_static_seperate_test.dart

 // Copyright (c) 2014, 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.
 
 // The class lib1.C is referenced via lib1
 // The static function lib1.C.foo is referenced via lib2
 // Dart2js will put them in seperate hunks.
 // Similarly for C2, ..., C5.
 
 import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
 import "deferred_static_seperate_lib1.dart" deferred as lib1;
 import "deferred_static_seperate_lib2.dart" deferred as lib2;
 
 void main() {
+  asyncStart();
   lib1.loadLibrary().then((_) {
     lib2.loadLibrary().then((_) {
+      print("HERE");
       Expect.equals(1, new lib1.C().bar());
       var x = new lib1.C2();
       Expect.mapEquals({1: 2}, x.bar);
       x.bar = {2: 3};
       Expect.mapEquals({2: 3}, x.bar);
 
       Expect.equals(lib1.x, new lib1.C3().bar);
       Expect.mapEquals({lib1.x: lib1.x}, new lib1.C4().bar);
       Expect.equals(1, new lib1.C5().bar());
 
       lib2.foo();
+      asyncEnd();
     });
   });
 }