[observe] optimize ObservableMap setter if no observers are present

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.
 
 library observe.src.observable_map;
 
 import 'dart:collection';
 import 'package:observe/observe.dart';
 
 
@@ skipping 87 matching lines @@
 
   @reflectable bool get isNotEmpty => !isEmpty;
 
   @reflectable bool containsValue(Object value) => _map.containsValue(value);
 
   @reflectable bool containsKey(Object key) => _map.containsKey(key);
 
   @reflectable V operator [](Object key) => _map[key];
 
   @reflectable void operator []=(K key, V value) {
+    if (!hasObservers) {
+      _map[key] = value;
+      return;
+    }
+
     int len = _map.length;
     V oldValue = _map[key];
+
     _map[key] = value;
-    if (hasObservers) {
-      if (len != _map.length) {
-        notifyPropertyChange(#length, len, _map.length);
-        notifyChange(new MapChangeRecord.insert(key, value));
-      } else if (oldValue != value) {
-        notifyChange(new MapChangeRecord(key, oldValue, value));
-      }
+
+    if (len != _map.length) {
+      notifyPropertyChange(#length, len, _map.length);
+      notifyChange(new MapChangeRecord.insert(key, value));
+    } else if (oldValue != value) {
+      notifyChange(new MapChangeRecord(key, oldValue, value));
     }
   }
 
   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);
@@ skipping 22 matching lines @@
       });
       notifyPropertyChange(#length, len, 0);
     }
     _map.clear();
   }
 
   void forEach(void f(K key, V value)) => _map.forEach(f);
 
   String toString() => Maps.mapToString(this);
 }