Make int an abstract class.

lib/core/int.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.
 
 // Dart core library.
 
-interface int extends num {
+abstract class int implements num {
   // Bit-operations.
   int operator &(int other);
   int operator |(int other);
   int operator ^(int other);
   int operator ~();
   int operator <<(int shiftAmount);
   int operator >>(int shiftAmount);
 
   // Testers.
   bool isEven();
   bool isOdd();
 
   // Specializations of super-interface.
   int operator -();
   int abs();
   int round();
   int floor();
   int ceil();
   int truncate();
 }