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

tests/compiler/dart2js/link_helper.dart

 // Copyright (c) 2014, 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 link_helper;
 
-import 'package:compiler/src/util/util.dart';
-import 'package:compiler/src/util/util_implementation.dart';
+import 'package:compiler/implementation/util/util.dart';
+import 'package:compiler/implementation/util/util_implementation.dart';
 
 Link LinkFromList(List list) {
   switch (list.length) {
     case 0:
       return new Link();
     case 1:
       return new LinkEntry(list[0]);
     case 2:
       return new LinkEntry(list[0], new LinkEntry(list[1]));
     case 3:
       return new LinkEntry(
           list[0], new LinkEntry(list[1], new LinkEntry(list[2])));
   }
   Link link = new Link();
   for (int i = list.length; i > 0; i--) {
     link = link.prepend(list[i - 1]);
   }
   return link;
 }