Make isEmpty a getter.

tests/html/localstorage_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('LocalStorageTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
@@ skipping 84 matching lines @@
         window.localStorage.getValues());
   });
 
   testWithLocalStorage('length', () {
     Expect.equals(3, window.localStorage.length);
     window.localStorage.clear();
     Expect.equals(0, window.localStorage.length);
   });
 
   testWithLocalStorage('isEmpty', () {
-    Expect.isFalse(window.localStorage.isEmpty());
+    Expect.isFalse(window.localStorage.isEmpty);
     window.localStorage.clear();
-    Expect.isTrue(window.localStorage.isEmpty());
+    Expect.isTrue(window.localStorage.isEmpty);
   });
 }