Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/dart_backend/renamer.dart

 // Copyright (c) 2012, 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 dart_backend;
 
 Comparator get _compareNodes =>
     compareBy((n) => n.getBeginToken().charOffset);
 
 abstract class Renamable implements Comparable {
@@ skipping 342 matching lines @@
   MinifyingGenerator();
 
   String generate(String originalName, bool isForbidden(String name)) {
     String result;
     do {
       result = generateMiniId(index++);
     } while (isForbidden(result));
     return result;
   }
 }