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

pkg/observe/lib/src/path_observer.dart

Index: pkg/observe/lib/src/path_observer.dart
diff --git a/pkg/observe/lib/src/path_observer.dart b/pkg/observe/lib/src/path_observer.dart
index 6384f6ee4735851af401ca70b290b348f9a9b7d0..6bdc3dfc286cb61adbef65a1ff6a54177e6bafa3 100644
--- a/pkg/observe/lib/src/path_observer.dart
+++ b/pkg/observe/lib/src/path_observer.dart
@@ -184,15 +184,28 @@ class PathObserver extends ChangeNotifier {
 
   void _observeIndex(int i) {
     final object = _values[i];
-    if (object is Observable) {
+    final segment = _segments[i];
+    if (segment is int) {
+      if (object is ObservableList) {
+        _subs[i] = object.listChanges.listen((List<ListChangeRecord> records) {
+          for (var record in records) {
+            if (record.indexChanged(segment)) {
+              _updateObservedValues(start: i);
+              return;
+            }
+          }
+        });
+      }
+    } else if (object is Observable) {
       // TODO(jmesserly): rather than allocating a new closure for each
       // property, we could try and have one for the entire path. However we'd
       // need to do a linear scan to find the index as soon as we got a change.
       // Also we need to fix ListChangeRecord and MapChangeRecord to contain
       // the target. Not sure if it's worth it.
+
       _subs[i] = object.changes.listen((List<ChangeRecord> records) {
         for (var record in records) {
-          if (_changeRecordMatches(record, _segments[i])) {
+          if (_changeRecordMatches(record, segment)) {
             _updateObservedValues(start: i);
             return;
           }
@@ -203,9 +216,6 @@ class PathObserver extends ChangeNotifier {
 }
 
 bool _changeRecordMatches(record, key) {
-  if (record is ListChangeRecord) {
-    return key is int && (record as ListChangeRecord).indexChanged(key);
-  }
   if (record is PropertyChangeRecord) {
     return (record as PropertyChangeRecord).name == key;
   }