[observe] add isEmpty/isNotEmpty to ObservableList reflection

pkg/observe/test/observable_list_test.dart

Index: pkg/observe/test/observable_list_test.dart
diff --git a/pkg/observe/test/observable_list_test.dart b/pkg/observe/test/observable_list_test.dart
index 0b4575dd1295c62cdad29aa58edd61ca9e884f82..66a03d343df7110e85cc31d79faa74d25b074ca0 100644
--- a/pkg/observe/test/observable_list_test.dart
+++ b/pkg/observe/test/observable_list_test.dart
@@ -253,7 +253,11 @@ main() {
       expect(list, []);
 
       performMicrotaskCheckpoint();
-      expectChanges(propRecords, [_lengthChange(6, 0)]);
+      expectChanges(propRecords, [
+          _lengthChange(6, 0),
+          new PropertyChangeRecord(list, #isEmpty, false, true),
+          new PropertyChangeRecord(list, #isNotEmpty, true, false),
+      ]);
       expectChanges(listRecords, [_change(0, removed: [1, 2, 3, 1, 3, 4])]);
     });
   });