Allow ChangeNotifier to be mixed in again (flutter#23631)

Luckily this class didn't actually need to extend its superclass, it
only implements the interface. So we can change `extends` to
`implements` and that's close enough, while allowing the class to be
mixed in again.

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -99,7 +99,7 @@ abstract class ValueListenable<T> extends Listenable {
 /// See also:
 ///
 ///  * [ValueNotifier], which is a [ChangeNotifier] that wraps a single value.
-class ChangeNotifier extends Listenable {
+class ChangeNotifier implements Listenable {
   ObserverList<VoidCallback> _listeners = ObserverList<VoidCallback>();
 
   bool _debugAssertNotDisposed() {

packages/flutter/test/foundation/change_notifier_test.dart

@@ -11,6 +11,24 @@ class TestNotifier extends ChangeNotifier {
   }
 }
 
+class HasListenersTester<T> extends ValueNotifier<T> {
+  HasListenersTester(T value) : super(value);
+  bool get testHasListeners => hasListeners;
+}
+
+class A {
+  bool result = false;
+  void test() { result = true; }
+}
+
+class B extends A with ChangeNotifier {
+  @override
+  void test() {
+    notifyListeners();
+    super.test();
+  }
+}
+
 void main() {
   testWidgets('ChangeNotifier', (WidgetTester tester) async {
     final List<String> log = <String>[];
@@ -258,9 +276,18 @@ void main() {
     notifier.removeListener(test2);
     expect(notifier.testHasListeners, isFalse);
   });
-}
 
-class HasListenersTester<T> extends ValueNotifier<T> {
-  HasListenersTester(T value) : super(value);
-  bool get testHasListeners => hasListeners;
+  test('ChangeNotifier as a mixin', () {
+    // We document that this is a valid way to use this class.
+    final B b = B();
+    int notifications = 0;
+    b.addListener(() {
+      notifications += 1;
+    });
+    expect(b.result, isFalse);
+    expect(notifications, 0);
+    b.test();
+    expect(b.result, isTrue);
+    expect(notifications, 1);
+  });
 }