Specs: rename sky:core to dart:sky

sky/specs/animation.md

 Animation API
 =============
 
 ```dart
 typedef void TimerCallback();
 
 class AnimationTimer extends Timer {
   external factory whenIdle(TimerCallback callback, { double budget: 1.0 });
   // calls callback next time the system is idle
   // - if budget is in the range 0.0 < budget <= 1.0, then callback is
@@ skipping 14 matching lines @@
   // are all canceled, so isActive will become false
 
 }
 ```
 
 
 Easing Functions
 ----------------
 
 ```dart
-// part of the framework, not sky:core
+// part of the framework, not dart:sky
 
 typedef void AnimationCallback();
 
 abstract class EasingFunction {
   EasingFunction({double duration: 0.0, AnimationCallback completionCallback: null });
   double getFactor(Float time);
   // calls completionCallback if time >= duration
   // then returns a number ostensibly in the range 0.0 to 1.0
   // (but it could in practice go outside this range, e.g. for
   // animation styles that overreach then come back)
 }
 ```
 
 If you want to have two animations simultaneously, e.g. two
 transforms, then you can add to the RenderNode's overrideStyles a
 StyleValue that combines other StyleValues, e.g. a
 "TransformStyleValueCombinerStyleValue", and then add to it the
 regular animated StyleValues, e.g. multiple
 "AnimatedTransformStyleValue" objects. A framework API could make
 setting all that up easy, given the right underlying StyleValue
 classes.