Use library prefixes in scope of the mixin class when compiling a functions of

tests/language/mixin_prefix_lib.dart

Index: tests/language/mixin_prefix_lib.dart
===================================================================
--- tests/language/mixin_prefix_lib.dart	(revision 0)
+++ tests/language/mixin_prefix_lib.dart	(revision 0)
@@ -0,0 +1,11 @@
+// Copyright (c) 2013, 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 mixin_prefix_lib;
+
+import 'dart:json' as json;
+
+class MixinClass {
+  String bar() => json.stringify({'a':1});
+}