Adding isNotEmpty property to collection and string.

samples/swarm/swarm_ui_lib/observable/observable.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.
 
 library observable;
 
 import 'dart:collection';
 
 part 'ChangeEvent.dart';
 part 'EventBatch.dart';
@@ skipping 307 matching lines @@
     return _internal.singleWhere(test);
   }
   T elementAt(int index) {
     return _internal.elementAt(index);
   }
   Map<int, T> asMap() {
     return _internal.asMap();
   }
 
   bool get isEmpty => length == 0;
+
+  bool get isNotEmpty => !isEmpty;
 }
 
 // TODO(jmesserly): is this too granular? Other similar systems make whole
 // classes observable instead of individual fields. The memory cost of having
 // every field effectively boxed, plus having a listeners list is likely too
 // much. Also, making a value observable necessitates adding ".value" to lots
 // of places, and constructing all fields with the verbose
 // "new ObservableValue<DataType>(myValue)".
 /** A wrapper around a single value whose change can be observed. */
 class ObservableValue<T> extends AbstractObservable {
   ObservableValue(T value, [Observable parent = null])
     : super(parent), _value = value;
 
   T get value => _value;
 
   void set value(T newValue) {
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }