Fix remaining analyzer warnings in Sky

sky/sdk/lib/animation/animation_performance.dart

Index: sky/sdk/lib/animation/animation_performance.dart
diff --git a/sky/sdk/lib/animation/animation_performance.dart b/sky/sdk/lib/animation/animation_performance.dart
index c730d8011cb5128856d95f2f743b9829c5d70a45..f4179e95ac770b515ba3487261be9e1cefff3b19 100644
--- a/sky/sdk/lib/animation/animation_performance.dart
+++ b/sky/sdk/lib/animation/animation_performance.dart
@@ -11,7 +11,7 @@ abstract class AnimatedVariable {
   void setFraction(double t);
 }
 
-class AnimatedType<T> extends AnimatedVariable {
+class AnimatedType<T extends dynamic> extends AnimatedVariable {
   T value;
   final T begin, end;
   final Curve curve;