Fix some checked mode errors in drawer.dart

We need to use the more generic type Curve. Also, added an assert to catch this
earlier.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1017463002

framework/animation/generators.dart

@@ -77,6 +77,7 @@ class AnimationGenerator extends Generator {
     this.curve: linear,
     Function onDone
   }) {
+    assert(curve != null);
     assert(duration != null && duration > 0.0);
     _generator = new FrameGenerator(onDone: onDone);
 

framework/components/drawer.dart

@@ -16,7 +16,7 @@ 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;