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

pkg/observe/lib/src/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.path_observer;
 
 import 'dart:async';
 @MirrorsUsed(metaTargets: const [Reflectable, ObservableProperty],
     override: 'observe.src.path_observer')
 import 'dart:mirrors';
@@ skipping 166 matching lines @@
     if (end == null) end = _segments.length;
 
     for (int i = start; i < end; i++) {
       if (_subs[i] != null) _subs[i].cancel();
       _observeIndex(i);
     }
   }
 
   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;
           }
         }
       });
     }
   }
 }
 
 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;
   }
   if (record is MapChangeRecord) {
     if (key is Symbol) key = MirrorSystem.getName(key);
     return (record as MapChangeRecord).key == key;
   }
   return false;
 }
 
@@ skipping 128 matching lines @@
 
 bool _isPathValid(String s) {
   s = s.replaceAll(_spacesRegExp, '');
 
   if (s == '') return true;
   if (s[0] == '.') return false;
   return _pathRegExp.hasMatch(s);
 }
 
 final _logger = new Logger('observe.PathObserver');