Decorate mixin application class name with private keys of libraries of super

tests/language/regress_27957_test.dart

+// Copyright (c) 2016, 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 'regress_27957_lib1.dart' as s1;
+import 'regress_27957_lib2.dart' as s2;
+
+class Mixin {}
+class C1 = s1.Superclass with Mixin;
+class C2 = s2.Superclass with Mixin;
+
+main() {
+  var c1 = new C1(), c2 = new C2();
+  Expect.equals(1, c1.m());
+  Expect.equals(2, c2.m());
+}