Fix (bogus) warnings related to the use of Function in dart2js.

sdk/lib/_internal/compiler/implementation/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;
 
-Function get _compareNodes =>
+Comparator get _compareNodes =>
     compareBy((n) => n.getBeginToken().charOffset);
 
 typedef String _Renamer(Renamable renamable);
 abstract class Renamable {
   final int RENAMABLE_TYPE_ELEMENT = 1;
   final int RENAMABLE_TYPE_MEMBER = 2;
   final int RENAMABLE_TYPE_LOCAL = 3;
 
   final Set<Node> nodes;
   final _Renamer renamer;
@@ skipping 331 matching lines @@
     index ~/= firstCharAlphabet.length;
     int length = otherCharsAlphabet.length;
     while (index >= length) {
       resultBuilder.write(otherCharsAlphabet[index % length]);
       index ~/= length;
     }
     resultBuilder.write(otherCharsAlphabet[index]);
     return resultBuilder.toString();
   }
 }