Fix assert in int64 and add test

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 776 matching lines @@
 
       q = d1 ~/ fatRadix;
       r = d1 - q * fatRadix;
       d1 = q;
       d0 += r << 10;
 
       q = d0 ~/ fatRadix;
       r = d0 - q * fatRadix;
       d0 = q;
 
-      assert(chunk2 == "");
+      assert(chunk3 == "");
       chunk3 = chunk2;
       chunk2 = chunk1;
       // Adding [fatRadix] Forces an extra digit which we discard to get a fixed
       // width.  E.g.  (1000000 + 123) -> "1000123" -> "000123".  An alternative
       // would be to pad to the left with zeroes.
       chunk1 = (fatRadix + r).toRadixString(radix).substring(1);
     }
     int residue = (d2 << 20) + (d1 << 10) + d0;
     String leadingDigits = residue == 0 ? '' : residue.toRadixString(radix);
     return '$sign$leadingDigits$chunk1$chunk2$chunk3';
@@ skipping 385 matching lines @@
         }
       }
       return ZERO;
     }
 
     // Generate the quotient using bit-at-a-time long division.
     return _divModHelper(aIsCopy ? a : new Int64._copy(a), b, negative,
         aIsNegative, aIsMinValue, computeRemainder);
   }
 }