Baby steps towards an odeon-like animation system. First victim: Drawer.

sky/sdk/lib/painting/box_painter.dart

Index: sky/sdk/lib/painting/box_painter.dart
diff --git a/sky/sdk/lib/painting/box_painter.dart b/sky/sdk/lib/painting/box_painter.dart
index 6f3a9d6e4995a4720473d53c84ac01c96d20af4e..d54cae65727eb47a7450e3e3e7336f62b135b5a3 100644
--- a/sky/sdk/lib/painting/box_painter.dart
+++ b/sky/sdk/lib/painting/box_painter.dart
@@ -6,6 +6,7 @@ import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:sky' show Point, Offset, Size, Rect, Color, Paint, Path;
 
+import '../base/lerp.dart';
 import 'shadows.dart';
 import 'package:sky/mojo/net/image_cache.dart' as image_cache;
 
@@ -72,6 +73,11 @@ class BoxShadow {
   String toString() => 'BoxShadow($color, $offset, $blur)';
 }
 
+BoxShadow lerpBoxShadow(BoxShadow a, BoxShadow b, double t) =>
+  new BoxShadow(color: lerpColor(a.color, b.color, t),
+                offset: lerpOffset(a.offset, b.offset, t),
+                blur: lerpNum(a.blur, b.blur, t));
+
 abstract class Gradient {
   sky.Shader createShader();
 }