Remove dynamic parameters from Observatory helpers

runtime/observatory/lib/src/elements/helpers/rendering_scheduler.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:observatory/src/elements/helpers/rendering_queue.dart';
 export 'package:observatory/src/elements/helpers/rendering_queue.dart';
 
 /// A generic renderable object.
@@ skipping 73 matching lines @@
   /// Set the object as dirty. A rendering will be scheduled.
   void dirty() {
     if (_dirty) return;
     _dirty = true;
     scheduleRendering();
   }
 
   /// Checks for modification during attribute set.
   /// If value changes a new rendering is scheduled.
   /// set attr(T v) => _attr = _r.checkAndReact(_attr, v);
-  dynamic checkAndReact(dynamic oldValue, dynamic newValue) {
+  T checkAndReact<T>(T oldValue, T newValue) {
     if (oldValue != newValue)
       dirty();
     else
       scheduleNotification();
     return newValue;
   }
 
   /// Schedules a new rendering phase.
   void scheduleRendering() {
     if (_renderingScheduled) return;
@@ skipping 25 matching lines @@
     _wait.addAll(it);
   }
 
   Future _notify() async {
     await Future.wait(_wait);
     _wait.clear();
     _onRendered.add(new RenderedEvent<T>(element, _dirty));
     _notificationScheduled = false;
   }
 }