Fix js test for firefox now that null becomes 'null' and not 'undefined'

sdk/lib/js/dart2js/js_dart2js.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 437 matching lines @@
   void setRange(int start, int end, Iterable<E> iterable, [int skipCount = 0]) {
     _checkRange(start, end, length);
     int length = end - start;
     if (length == 0) return;
     if (skipCount < 0) throw new ArgumentError(skipCount);
     var args = [start, length]..addAll(iterable.skip(skipCount).take(length));
     callMethod('splice', args);
   }
 
   void sort([int compare(E a, E b)]) {
-    callMethod('sort', [compare]);
+    // Note: arr.sort(null) is a type error in FF
+    callMethod('sort', compare == null ? [] : [compare]);
   }
 }
 
 // property added to a Dart object referencing its JS-side DartObject proxy
 final String _DART_OBJECT_PROPERTY_NAME =
     getIsolateAffinityTag(r'_$dart_dartObject');
 final String _DART_CLOSURE_PROPERTY_NAME =
     getIsolateAffinityTag(r'_$dart_dartClosure');
 
 // property added to a JS object referencing its Dart-side JsObject proxy
@@ skipping 105 matching lines @@
   // Dart in that context. The JS object will have a cached proxy
   // but it won't be a valid Dart object in this context.
   // For now we throw away the cached proxy, but we should be able
   // to cache proxies from multiple JS contexts and Dart isolates.
   if (dartProxy == null || !_isLocalObject(o)) {
     dartProxy = createProxy(o);
     _defineProperty(o, propertyName, dartProxy);
   }
   return dartProxy;
 }