Mixin tests with libraries

tests/language/mixin_lib_extends_field_lib.dart

Index: tests/language/mixin_lib_extends_field_lib.dart
===================================================================
--- tests/language/mixin_lib_extends_field_lib.dart	(revision 0)
+++ tests/language/mixin_lib_extends_field_lib.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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_lib_extends_field_lib;
+
+class M1 {
+  final bar = "M1-bar";
+}
+
+class M2 {
+  var baz = "M2-$_baz";
+}
+
+var _baz = "baz";