package:observe fix various api issues

pkg/observe/lib/src/observable_box.dart

Index: pkg/observe/lib/src/observable_box.dart
diff --git a/pkg/observe/lib/src/observable_box.dart b/pkg/observe/lib/src/observable_box.dart
index 1bdf4cced3fcc70b68ffd7ec77b2aaec33649943..7db72bfb8b893b176fb142ddbbe65dbe3e6aac2b 100644
--- a/pkg/observe/lib/src/observable_box.dart
+++ b/pkg/observe/lib/src/observable_box.dart
@@ -10,9 +10,9 @@ part of observe;
  * An observable box that holds a value. Use this if you want to store a single
  * value. For other cases, it is better to use [ObservableList],
  * [ObservableMap], or a custom [Observable] implementation based on
- * [ObservableMixin]. The property name for changes is "value".
+ * [Observable]. The property name for changes is "value".
  */
-class ObservableBox<T> extends ChangeNotifierBase {
+class ObservableBox<T> extends ChangeNotifier {
   T _value;
 
   ObservableBox([T initialValue]) : _value = initialValue;