dart2js: fix mixin support in deferred libraries.

tests/language/deferred_mixin_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.
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+
+import "deferred_mixin_lib1.dart" deferred as lib1;
+import "deferred_mixin_lib2.dart" deferred as lib2;
+
+class NonDeferredMixin {
+ foo() => "NonDeferredMixin";
+}
+
+main() {
+  Expect.equals("NonDeferredMixin", new NonDeferredMixin().foo());
+  asyncStart();
+  lib1.loadLibrary().then((_) {
+    Expect.equals("lib1.Mixin", new lib1.Mixin().foo());
+    Expect.equals("A with NonDeferredMixin", new lib1.A().foo());
+    Expect.equals("B with lib1.Mixin", new lib1.B().foo());
+    Expect.equals("C with NonDeferredMixin", new lib1.C().foo());
+    Expect.equals("D with lib1.Mixin", new lib1.D().foo());
+    Expect.equals("E with SharedMixin", new lib1.E().foo());
+    lib2.loadLibrary().then((_) {
+      Expect.equals("lib2.A with SharedMixin", new lib2.A().foo());
+      asyncEnd();
+    });
+  });
+}