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

pkg/polymer_expressions/lib/eval.dart

Index: pkg/polymer_expressions/lib/eval.dart
diff --git a/pkg/polymer_expressions/lib/eval.dart b/pkg/polymer_expressions/lib/eval.dart
index c12d4d12a5d45c352dd5d8807876a28c705601b3..b774807d05d0665b57a0b69f230a5cce5e5b2a49 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -589,12 +589,7 @@ class InObserver extends ExpressionObserver<InExpression>
     }
 
     if (iterable is ObservableList) {
-      _subscription = (iterable as ObservableList).changes.listen(
-          (List<ChangeRecord> changes) {
-            if (changes.any((c) => c is ListChangeRecord)) {
-              _invalidate(scope);
-            }
-          });
+      _subscription = iterable.listChanges.listen((_) => _invalidate(scope));
     }
 
     // TODO: make Comprehension observable and update it