Revert "Remove NullPointerException."

runtime/lib/expando_patch.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.
 
 patch class Expando<T> {
   /* patch */ Expando([this.name]) : _data = new List();
 
   /* patch */ T operator[](Object object) {
     _checkType(object);
     var doCompact = false;
@@ skipping 36 matching lines @@
       _data[i].value = value;
     } else {
       _data.add(new _WeakProperty(object, value));
     }
     if (doCompact) {
       _data = _data.filter((e) => (e != null));
     }
   }
 
   static _checkType(object) {
-    if (object == null || object is bool || object is num || object is String) {
+    if (object == null) {
+      throw new NullPointerException();
+    }
+    if (object is bool || object is num || object is String) {
       throw new ArgumentError(object);
     }
   }
 
   List _data;
 }