Adding isNotEmpty property to collection and string.

sdk/lib/collection/hash_map.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.
 
 part of dart.collection;
 
 class HashMap<K, V> implements Map<K, V> {
   external HashMap();
 
   factory HashMap.from(Map<K, V> other) {
     return new HashMap<K, V>()..addAll(other);
   }
 
   external int get length;
   external bool get isEmpty;
+  external bool get isNotEmpty;
 
   external Iterable<K> get keys;
   external Iterable<V> get values;
 
   external bool containsKey(K key);
   external bool containsValue(V value);
 
   external void addAll(Map<K, V> other);
 
   external V operator [](K key);
   external void operator []=(K key, V value);
 
   external V putIfAbsent(K key, V ifAbsent());
 
   external V remove(K key);
   external void clear();
 
   external void forEach(void action(K key, V value));
 
   String toString() => Maps.mapToString(this);
 }