Make isEmpty a getter.

tests/corelib/hash_map_test.dart

 // Copyright (c) 2011, 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.
 
 // Test program for the HashMap class.
 
 class HashMapTest {
 
   static testMain() {
     // TODO(srdjan/ngeoffray): Add more meaningful testing below. For now this
     // is used to verify that the test script is picking up these tests.
     var m = new Map();
     Expect.equals(0, m.length);
-    Expect.equals(true, m.isEmpty());
+    Expect.equals(true, m.isEmpty);
     m["one"] = 1;
     Expect.equals(1, m.length);
-    Expect.equals(false, m.isEmpty());
+    Expect.equals(false, m.isEmpty);
     Expect.equals(1, m["one"]);
   }
 }
 
 main() {
   HashMapTest.testMain();
 }