Work around for private names in mixin applications.

test/kernel/regression/mixin_library.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.md file.
 
 library test.mixin_library;
 
 f() => 2;
 
 V() => 87;
 
 _private() => 117;
 
 class Mixin<T> {
   var x = f(), y, z;
   T t;
   foo() => super.foo() + f();
   T g(T a) => null;
   h() => V();
   l() => _private();
+  _privateMethod() => 49;
+  publicMethod() => _privateMethod();
 }
+
+foo(m) => m._privateMethod();