Fix some checked mode errors in drawer.dart

sky/framework/components/drawer.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 '../animation/animation.dart';
 import '../animation/curves.dart';
 import '../fn.dart';
 import '../theme/colors.dart';
 import '../theme/shadows.dart';
 import 'dart:async';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'material.dart';
 
 const double _kWidth = 304.0;
 const double _kMinFlingVelocity = 0.4;
 const double _kBaseSettleDurationMS = 246.0;
 const double _kMaxSettleDurationMS = 600.0;
-const Cubic _kAnimationCurve = parabolicRise;
+const Curve _kAnimationCurve = parabolicRise;
 
 class DrawerAnimation extends Animation {
   Stream<double> get onPositionChanged => onValueChanged;
 
   bool get _isMostlyClosed => value <= -_kWidth / 2;
 
   DrawerAnimation() {
     value = -_kWidth;
   }
 
@@ skipping 131 matching lines @@
       level: level
     );
 
     return new Container(
       style: _style,
       inlineStyle: inlineStyle,
       children: [ mask, content ]
     );
   }
 }