Revert "Remove NullPointerException."

pkg/fixnum/test/int_32_test.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.
 
 library int32test;
 import '../lib/fixnum.dart';
 
 void main() {
   Expect.equals("0", new int32.fromInt(0).toString());
   Expect.equals("1", new int32.fromInt(1).toString());
@@ skipping 114 matching lines @@
 
   Expect.equals(new int32.fromInt(0x12345678 >> 7),
       new int32.fromInt(0x12345678) >> 7);
   Expect.equals(new int32.fromInt(0x12345678 << 7),
       new int32.fromInt(0x12345678) << 7);
   Expect.equals(new int32.fromInt(0x12345678 >> 7),
       new int32.fromInt(0x12345678).shiftRightUnsigned(7));
 
   try {
     new int32.fromInt(17) < null;
-    Expect.fail("x < null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x < null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   try {
     new int32.fromInt(17) <= null;
-    Expect.fail("x <= null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x <= null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   try {
     new int32.fromInt(17) > null;
-    Expect.fail("x > null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x > null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   try {
     new int32.fromInt(17) < null;
-    Expect.fail("x >= null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x >= null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   Expect.isFalse(new int32.fromInt(17) == null);
 }