Fix some checked mode errors in drawer.dart

sky/framework/animation/generators.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'curves.dart';
 import 'dart:async';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'mechanics.dart';
 
@@ skipping 59 matching lines @@
   bool _done = false;
 
   AnimationGenerator({
     this.initialDelay: 0.0,
     this.duration,
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
     Function onDone
   }) {
+    assert(curve != null);
     assert(duration != null && duration > 0.0);
     _generator = new FrameGenerator(onDone: onDone);
 
     double startTime = 0.0;
     _stream = _generator.onTick.map((timeStamp) {
       if (startTime == 0.0)
         startTime = timeStamp;
 
       double t = (timeStamp - (startTime + initialDelay)) / duration;
       return math.max(0.0, math.min(t, 1.0));
@@ skipping 53 matching lines @@
   double _update(double timeStamp) {
     double previousTime = _previousTime;
     _previousTime = timeStamp;
     if (previousTime == 0.0)
       return timeStamp;
     double deltaT = timeStamp - previousTime;
     system.update(deltaT);
     return timeStamp;
   }
 }