Fix list_path_observer, temporarily add a test for it, but we'll likely remove

pkg/observe/lib/src/list_path_observer.dart

Index: pkg/observe/lib/src/list_path_observer.dart
diff --git a/pkg/observe/lib/src/list_path_observer.dart b/pkg/observe/lib/src/list_path_observer.dart
index 44b2759d47f427e362e57e8539d76235be49aae9..bab01c230fd5d121ae9ef534fe2fb8d9d27f9a49 100644
--- a/pkg/observe/lib/src/list_path_observer.dart
+++ b/pkg/observe/lib/src/list_path_observer.dart
@@ -16,7 +16,6 @@ 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;
@@ -40,8 +39,8 @@ class ListPathObserver<E, P> extends ChangeNotifier {
 
   void dispose() {
     if (_sub != null) _sub.cancel();
-    _subs.forEach((s) => s.cancel());
-    _subs.clear();
+    _observers.forEach((o) => o.close());
+    _observers.clear();
   }
 
   void _reduce() {
@@ -61,15 +60,13 @@ class ListPathObserver<E, P> extends ChangeNotifier {
       for (int i = 0; i < lengthAdjust; i++) {
         int len = _observers.length;
         var pathObs = new PathObserver(list, '$len.$_itemPath');
-        _subs.add(pathObs.changes.listen(_scheduleReduce));
+        pathObs.open(_scheduleReduce);
         _observers.add(pathObs);
       }
     } else if (lengthAdjust < 0) {
       for (int i = 0; i < -lengthAdjust; i++) {
-        _subs.removeLast().cancel();
+        _observers.removeLast().close();
       }
-      int len = _observers.length;
-      _observers.removeRange(len + lengthAdjust, len);
     }
   }
 }