Adding isNotEmpty property to collection and string.

pkg/serialization/lib/src/serialization_helpers.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.
 
 /**
  * This contains extra functions and classes useful for implementing
  * serialiation. Some or all of these will be removed once the functionality is
  * available in the core library.
  */
 library serialization_helpers;
@@ skipping 230 matching lines @@
     keys.removeAt(index);
     return values.removeAt(index);
   }
 
   int get length => keys.length;
   void clear() {
     keys.clear();
     values.clear();
   }
   bool get isEmpty => keys.isEmpty;
+  bool get isNotEmpty => !isEmpty;
 
   // Note that this is doing an equality comparison.
   bool containsValue(x) => values.contains(x);
 }