Propagate notifications when onNotification returns null (#10081)

Previously we threw an exception to require a non-null value. Now we
just treat null as the common case of not stopping propagation.

packages/flutter/lib/src/widgets/notification_listener.dart

@@ -98,8 +98,8 @@ class NotificationListener<T extends Notification> extends StatelessWidget {
   /// Called when a notification of the appropriate type arrives at this
   /// location in the tree.
   ///
-  /// Return true to cancel the notification bubbling. Return false to allow the
-  /// notification to continue to be dispatched to further ancestors.
+  /// Return true to cancel the notification bubbling. Return false (or null) to
+  /// allow the notification to continue to be dispatched to further ancestors.
   ///
   /// The notification's [Notification.visitAncestor] method is called for each
   /// ancestor, and invokes this callback as appropriate.
@@ -108,21 +108,7 @@ class NotificationListener<T extends Notification> extends StatelessWidget {
   bool _dispatch(Notification notification, Element element) {
     if (onNotification != null && notification is T) {
       final bool result = onNotification(notification);
-      assert(() {
-        if (result == null)
-          throw new FlutterError(
-            'NotificationListener<$T> handler returned null.\n'
-            'The onNotification handler for the NotificationListener with the '
-            'following element returned null:\n'
-            '  $element\n'
-            'The ancestor chain for this widget was as follows:\n'
-            '  ${element.debugGetCreatorChain(12)}\n'
-            'Notification listeners must return true to stop the notification bubbling, '
-            'or false to allow it to continue (the common case is returning false).'
-          );
-        return true;
-      });
-      return result;
+      return result == true; // so that null and false have the same effect
     }
     return false;
   }

packages/flutter/test/widgets/notification_test.dart

@@ -61,11 +61,18 @@ void main() {
   });
 
   testWidgets('Notification basics - listener null return value', (WidgetTester tester) async {
+    final List<Type> log = <Type>[];
     final GlobalKey key = new GlobalKey();
     await tester.pumpWidget(new NotificationListener<MyNotification>(
+      onNotification: (MyNotification value) {
+        log.add(value.runtimeType);
+      },
+      child: new NotificationListener<MyNotification>(
         onNotification: (MyNotification value) { },
         child: new Container(key: key),
+      ),
     ));
-    expect(() { new MyNotification().dispatch(key.currentContext); }, throwsAssertionError);
+    expect(() { new MyNotification().dispatch(key.currentContext); }, isNot(throwsException));
+    expect(log, <Type>[MyNotification]);
   });
 }