Refactor and simplify the dart2dart renamer.

tests/compiler/dart2js/mirror_helper_rename_test.dart

 // 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.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'memory_compiler.dart' show compilerFor;
 import 'package:compiler/implementation/apiimpl.dart' show
     Compiler;
@@ skipping 19 matching lines @@
   backend.useMirrorHelperLibrary = useMirrorHelperLibrary;
   return
       compiler.runCompiler(Uri.parse('memory:main.dart')).then((_) => compiler);
 }
 
 void testWithMirrorHelperLibrary({bool minify}) {
   asyncTest(() => runCompiler(useMirrorHelperLibrary: true, minify: minify).
       then((Compiler compiler) {
     DartBackend backend = compiler.backend;
     MirrorRenamer mirrorRenamer = backend.mirrorRenamer;
-    Map<Node, String> renames = backend.renames;
+    Map<Node, String> renames = backend.placeholderRenamer.renames;
     Map<String, String> symbols = mirrorRenamer.symbols;
 
     Expect.isFalse(null == backend.mirrorHelperLibrary);
     Expect.isFalse(null == backend.mirrorHelperGetNameFunction);
 
     for (Node n in renames.keys) {
       if (symbols.containsKey(renames[n])) {
         if(n.toString() == 'getName') {
           Expect.equals(
               MirrorRenamer.MIRROR_HELPER_GET_NAME_FUNCTION,
@@ skipping 39 matching lines @@
 class Foo {
   noSuchMethod(Invocation invocation) {
     MirrorSystem.getName(invocation.memberName);
   }
 }
 
 void main() {
   new Foo().fisk();
 }
 """};