Make isEmpty a getter.

lib/core/collection.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.
 
 /**
  * The common interface of all collections.
  *
  * The [Collection] class contains a skeleton implementation of
  * an iterator based collection.
  */
@@ skipping 77 matching lines @@
   bool some(bool f(E element)) {
     for (E element in this) {
       if (f(element)) return true;
     }
     return false;
   }
 
   /**
    * Returns true if there is no element in this collection.
    */
-  bool isEmpty() => !iterator().hasNext;
+  bool get isEmpty => !iterator().hasNext;
 }