dart2js: Use an abstract Name class for names in the generated JavaScript ast.

pkg/compiler/lib/src/js_backend/frequency_namer.dart

+// Copyright (c) 2015, 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.
+
+part of js_backend;
+
+class FrequencyNamer extends Namer {
+  FrequencyNamer(Compiler compiler) : super(compiler);
+
+  jsAst.Name getFreshName(String proposedName, Set<String> usedNames,
+      Map<String, String> suggestedNames,
+      {bool sanitizeForNatives: false, bool sanitizeForAnnotations: false}) {
+    return null;
+  }
+}