dart2js: fix mixin support in deferred libraries.

tests/language/deferred_mixin_lib2.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.
 
 library lib2;
-import 'deferred_mirrors2_lazy.dart' deferred as admin;
+
+import "deferred_mixin_shared.dart";
+
+class A extends Object with SharedMixin {
+  foo() {
+    return "lib2.A with " + super.foo();
+  }
+}