Revert "Remove NullPointerException."

pkg/fixnum/lib/src/int32.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 32-bit signed integer, in the range [-2^31, 2^31 - 1].
  * Arithmetic operations may overflow in order to maintain this range.
  */
@@ skipping 124 matching lines @@
   /**
    * Constructs an [int32] from an [int].  Only the low 32 bits of the input
    * are used.
    */
   int32.fromInt(int i) : _i = (i & 0x7fffffff) - (i & 0x80000000);
 
   // Convert an [int] or [intx] to an [int32].  Note that an [int64]
   // will be truncated.
   int _convert(other) {
     if (other == null) {
-      throw new ArgumentError("other is null");
+      throw new NullPointerException();
     } else if (other is intx) {
       return other.toInt32()._i;
     } else if (other is int) {
       return other;
     } else {
       throw new Exception("Can't retrieve 32-bit int from $other");
     }
   }
 
   // The +, -, * , &, |, and ^ operaters deal with types as follows:
@@ skipping 191 matching lines @@
   }
 
   int toInt() => _i;
   int32 toInt32() => this;
   int64 toInt64() => new int64.fromInt(_i);
 
   String toString() => _i.toString();
   String toHexString() => _i.toRadixString(16);
   String toRadixString(int radix) => _i.toRadixString(radix);
 }