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

pkg/compiler/lib/src/util/link_implementation.dart

 // Copyright (c) 2011, 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 util_implementation;
 
 class LinkIterator<T> implements Iterator<T> {
   T _current;
   Link<T> _link;
 
@@ skipping 164 matching lines @@
     if (head == null) {
       head = entry;
     } else {
       lastLink.tail = entry;
     }
     lastLink = entry;
   }
 
   bool get isEmpty => length == 0;
 }