introduce ObservableList.listChanges to keep list changes separate from property changes

pkg/observe/lib/src/list_path_observer.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.list_path_observer;
 
 import 'dart:async';
 import 'package:observe/observe.dart';
 
 // Inspired by ArrayReduction at:
 // https://raw.github.com/rafaelw/ChangeSummary/master/util/array_reduction.js
 // The main difference is we support anything on the rich Dart Iterable API.
 
 /**
  * Observes a path starting from each item in the list.
  */
 class ListPathObserver<E, P> extends ChangeNotifier {
   final ObservableList<E> list;
   final String _itemPath;
   final List<PathObserver> _observers = <PathObserver>[];
   final List<StreamSubscription> _subs = <StreamSubscription>[];
   StreamSubscription _sub;
   bool _scheduled = false;
   Iterable<P> _value;
 
   ListPathObserver(this.list, String path)
       : _itemPath = path {
 
-    _sub = list.changes.listen((records) {
+    // TODO(jmesserly): delay observation until we are observed.
+    _sub = list.listChanges.listen((records) {
       for (var record in records) {
-        if (record is ListChangeRecord) {
-          _observeItems(record.addedCount - record.removedCount);
-        }
+        _observeItems(record.addedCount - record.removed.length);
       }
       _scheduleReduce(null);
     });
 
     _observeItems(list.length);
     _reduce();
   }
 
   @reflectable Iterable<P> get value => _value;
 
@@ skipping 25 matching lines @@
       }
     } else if (lengthAdjust < 0) {
       for (int i = 0; i < -lengthAdjust; i++) {
         _subs.removeLast().cancel();
       }
       int len = _observers.length;
       _observers.removeRange(len + lengthAdjust, len);
     }
   }
 }