Refactor and simplify the dart2dart renamer.

tests/compiler/dart2js/mirror_helper_unique_minification_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 testUniqueMinification() {
   asyncTest(() => runCompiler(useMirrorHelperLibrary: true, minify: true).
       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;
 
     // Check that no two different source code names get the same mangled name,
     // with the exception of MirrorSystem.getName that gets renamed to the same
     // mangled name as the getNameHelper from _mirror_helper.dart.
     for (Node node in renames.keys) {
       Identifier identifier = node.asIdentifier();
       if (identifier != null) {
         String source = identifier.source;
         Send send = mirrorRenamer.mirrorSystemGetNameNodes.first;
         if (send.selector == node)
           continue;
         if (symbols.containsKey(renames[node])) {
           print(node);
           Expect.equals(source, symbols[renames[node]]);
         }
       }
     }
   }));
 }
 
 void testNoUniqueMinification() {
   asyncTest(() => runCompiler(useMirrorHelperLibrary: false, minify: true).
       then((Compiler compiler) {
     DartBackend backend = compiler.backend;
-    Map<Node, String> renames = backend.renames;
+    Map<Node, String> renames = backend.placeholderRenamer.renames;
 
     // 'Foo' appears twice and 'invocation' and 'hest' get the same mangled
     // name.
     Expect.equals(renames.values.toSet().length, renames.values.length - 2);
   }));
 }
 
 const MEMORY_SOURCE_FILES = const <String, String> {
   'main.dart': """
 import 'dart:mirrors';
 
 class Foo {
   noSuchMethod(invocation) {
     MirrorSystem.getName(null);
   }
 }
 
 main(hest) {
   new Foo().fisk();
 }
 """};