Revert "Remove NullPointerException."

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 224 matching lines @@
   int64.fromInts(int top, int bottom) {
     top &= 0xffffffff;
     bottom &= 0xffffffff;
     _l = bottom & _MASK;
     _m = ((top & 0xfff) << 10) | ((bottom >> _BITS) & 0x3ff);
     _h = (top >> 12) & _MASK_2;
   }
 
   int64 _promote(other) {
     if (other == null) {
-      throw new ArgumentError("other is null");
+      throw new NullPointerException();
     } else if (other is intx) {
       other = other.toInt64();
     } else if (other is int) {
       other = new int64.fromInt(other);
     }
     if (other is !int64) {
       throw new Exception("Can't promote $other to int64");
     }
     return other;
   }
@@ skipping 832 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);
   }
 }