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

pkg/observe/lib/src/change_notifier.dart

Index: pkg/observe/lib/src/change_notifier.dart
diff --git a/pkg/observe/lib/src/change_notifier.dart b/pkg/observe/lib/src/change_notifier.dart
index 133c6c88cb7c9a2ab9e744337045833debcdd10a..54653e9e99115313cbb133b61130bfa845539d16 100644
--- a/pkg/observe/lib/src/change_notifier.dart
+++ b/pkg/observe/lib/src/change_notifier.dart
@@ -40,7 +40,10 @@ abstract class ChangeNotifier implements Observable {
    * Override this method to be called when the [changes] are no longer being
    * observed.
    */
-  void unobserved() {}
+  void unobserved() {
+    // Free some memory
+    _changes = null;
+  }
 
   bool deliverChanges() {
     var records = _records;