dart2js: fix mixin support in deferred libraries.

tests/language/deferred_mixin_shared.dart

Index: tests/language/deferred_mixin_shared.dart
diff --git a/tests/compiler/dart2js/http_launch_data/http_launch_main.dart b/tests/language/deferred_mixin_shared.dart
similarity index 77%
copy from tests/compiler/dart2js/http_launch_data/http_launch_main.dart
copy to tests/language/deferred_mixin_shared.dart
index 68ea63472f29d5b7d7d4de8a863c4d05cd7db0bd..2ad98328b258a3823c713b76183921448a86b10e 100644
--- a/tests/compiler/dart2js/http_launch_data/http_launch_main.dart
+++ b/tests/language/deferred_mixin_shared.dart
@@ -2,9 +2,8 @@
 // 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 shared;
 
-import 'lib1.dart';
-
-main() {
-  print(foo());
-}
+class SharedMixin {
+  foo() => "SharedMixin";
+}