Delete dart_backend from compiler.

pkg/compiler/lib/src/mirror_renamer/mirror_renamer.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.
-
-library mirror_renamer;
-
-import '../compiler.dart' show Compiler;
-import '../dart_backend/dart_backend.dart'
-    show DartBackend, PlaceholderCollector;
-import '../elements/elements.dart';
-import '../tokens/token.dart' show Token;
-import '../tree/tree.dart';
-
-part 'renamer.dart';
-
-class MirrorRenamer {
-  const MirrorRenamer();
-
-  LibraryElement get helperLibrary => null;
-
-  FunctionElement get getNameFunction => null;
-
-  bool isMirrorHelperLibrary(LibraryElement element) => false;
-
-  void registerStaticSend(Element currentElement, Element target, Node node) {}
-
-  void addRenames(Map<Node, String> renames, List<Node> topLevelNodes,
-      PlaceholderCollector placeholderCollector) {}
-}