use symbol literals instead of const ctor in packages

pkg/observe/lib/src/observable_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 observe;
 
 // TODO(jmesserly): this needs to be faster. We currently require multiple
 // lookups per key to get the old value.
 // TODO(jmesserly): this doesn't implement the precise interfaces like
 // LinkedHashMap, SplayTreeMap or HashMap. However it can use them for the
@@ skipping 26 matching lines @@
     return '#<MapChangeRecord $kind $key>';
   }
 }
 
 /**
  * Represents an observable map of model values. If any items are added,
  * removed, or replaced, then observers that are listening to [changes]
  * will be notified.
  */
 class ObservableMap<K, V> extends ChangeNotifierBase implements Map<K, V> {
-  static const _LENGTH = const Symbol('length');
-
   final Map<K, V> _map;
 
   /** Creates an observable map. */
   ObservableMap() : _map = new HashMap<K, V>();
 
   /** Creates a new observable map using a [LinkedHashMap]. */
   ObservableMap.linked() : _map = new LinkedHashMap<K, V>();
 
   /** Creates a new observable map using a [SplayTreeMap]. */
   ObservableMap.sorted() : _map = new SplayTreeMap<K, V>();
@@ skipping 38 matching lines @@
   bool containsKey(Object key) => _map.containsKey(key);
 
   V operator [](Object key) => _map[key];
 
   void operator []=(K key, V value) {
     int len = _map.length;
     V oldValue = _map[key];
     _map[key] = value;
     if (hasObservers) {
       if (len != _map.length) {
-        notifyPropertyChange(_LENGTH, len, _map.length);
+        notifyPropertyChange(#length, len, _map.length);
         notifyChange(new MapChangeRecord(key, isInsert: true));
       } else if (!identical(oldValue, value)) {
         notifyChange(new MapChangeRecord(key));
       }
     }
   }
 
   void addAll(Map<K, V> other) {
     other.forEach((K key, V value) { this[key] = value; });
   }
 
   V putIfAbsent(K key, V ifAbsent()) {
     int len = _map.length;
     V result = _map.putIfAbsent(key, ifAbsent);
     if (hasObservers && len != _map.length) {
-      notifyPropertyChange(_LENGTH, len, _map.length);
+      notifyPropertyChange(#length, len, _map.length);
       notifyChange(new MapChangeRecord(key, isInsert: true));
     }
     return result;
   }
 
   V remove(Object key) {
     int len = _map.length;
     V result =  _map.remove(key);
     if (hasObservers && len != _map.length) {
       notifyChange(new MapChangeRecord(key, isRemove: true));
-      notifyPropertyChange(_LENGTH, len, _map.length);
+      notifyPropertyChange(#length, len, _map.length);
     }
     return result;
   }
 
   void clear() {
     int len = _map.length;
     if (hasObservers && len > 0) {
       _map.forEach((key, value) {
         notifyChange(new MapChangeRecord(key, isRemove: true));
       });
-      notifyPropertyChange(_LENGTH, len, 0);
+      notifyPropertyChange(#length, len, 0);
     }
     _map.clear();
   }
 
   void forEach(void f(K key, V value)) => _map.forEach(f);
 
   String toString() => Maps.mapToString(this);
 }