Ensure that unmangled private names are unique.

tests/lib/mirrors/private_symbol_mangling_lib.dart

 // Copyright (c) 2014, 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.
 
-// Regression test for issue 18865.
+library other;
 
-class B<T> { }
+var _privateGlobalField = 3;
 
-class A<T> extends B {
-  static foo() => new A();
+_privateGlobalMethod() => 11;
+
+class C2 {
+  var _privateField = 1;
+  _privateMethod() => 3;
 }
-
-main () {
-  A.foo();
-}