Fix --strong warnings in pkg/fixnum

pkg/fixnum/lib/src/int64.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 fixnum;
 
 /**
  * An immutable 64-bit signed integer, in the range [-2^63, 2^63 - 1].
  * Arithmetic operations may overflow in order to maintain this range.
  */
@@ skipping 12 matching lines @@
 
   final int _l, _m, _h;
 
   // Note: several functions require _BITS == 22 -- do not change this value.
   static const int _BITS = 22;
   static const int _BITS01 = 44; // 2 * _BITS
   static const int _BITS2 = 20; // 64 - _BITS01
   static const int _MASK = 4194303; // (1 << _BITS) - 1
   static const int _MASK2 = 1048575; // (1 << _BITS2) - 1
   static const int _SIGN_BIT = 19; // _BITS2 - 1
-  static const int _SIGN_BIT_MASK = 524288; // 1 << _SIGN_BIT
+  static const int _SIGN_BIT_MASK = 1 << _SIGN_BIT;
 
   /**
    * The maximum positive value attainable by an [Int64], namely
    * 9,223,372,036,854,775,807.
    */
   static const Int64 MAX_VALUE = const Int64._bits(_MASK, _MASK, _MASK2 >> 1);
 
   /**
    * The minimum positive value attainable by an [Int64], namely
    * -9,223,372,036,854,775,808.
@@ skipping 949 matching lines @@
       if (r0 == 0 && r1 == 0 && r2 == 0) {
         return ZERO;
       } else {
         return _sub(b0, b1, b2, r0, r1, r2);
       }
     } else {
       return _negate(r0, r1, r2);
     }
   }
 }