Move diff.dart to dart2js_incremental.

dart/pkg/dart2js_incremental/lib/diff.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 trydart.poi.diff;
 
 import 'package:compiler/implementation/elements/elements.dart' show
     AbstractFieldElement,
     ClassElement,
     CompilationUnitElement,
@@ skipping 89 matching lines @@
       beforeToken = beforeToken.next;
       afterToken = afterToken.next;
       beforeKind = beforeToken.kind;
       afterKind = afterToken.kind;
     }
     return beforeKind != afterKind;
   }
   print("$before isn't a PartialElement");
   return true;
 }