dart2js: fix mixin support in deferred libraries.

tests/language/deferred_mixin_lib2.dart

Index: tests/language/deferred_mixin_lib2.dart
diff --git a/tests/compiler/dart2js_extra/deferred/deferred_mirrors2_lib2.dart b/tests/language/deferred_mixin_lib2.dart
similarity index 62%
copy from tests/compiler/dart2js_extra/deferred/deferred_mirrors2_lib2.dart
copy to tests/language/deferred_mixin_lib2.dart
index 1a16dafa3ac1fb68acc2702942f969ab2e6cbc06..afe81b8960c9dee7b197cda29172c563413265cd 100644
--- a/tests/compiler/dart2js_extra/deferred/deferred_mirrors2_lib2.dart
+++ b/tests/language/deferred_mixin_lib2.dart
@@ -3,4 +3,11 @@
 // 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();
+  }
+}