Implement PointerEvent#dx

sky/framework/sky-drawer.sky

Index: sky/framework/sky-drawer.sky
diff --git a/sky/framework/sky-drawer.sky b/sky/framework/sky-drawer.sky
index 30e4e3e1e294731e61084d8a520033bb23e33d93..12c3b2bceff0835d908348feb3c458c5dba33105 100644
--- a/sky/framework/sky-drawer.sky
+++ b/sky/framework/sky-drawer.sky
@@ -48,7 +48,6 @@ const kAnimationCurve = Curves.easeInOut;
 
 module.exports = class extends SkyElement {
   created() {
-    this.previousX_ = 0;
     this.position_ = 0;
     this.mask_ = null;
     this.content_ = null;
@@ -125,14 +124,10 @@ module.exports = class extends SkyElement {
 
   handlePointerDown(event) {
     this.animation_.stop();
-    this.previousX_ = event.x;
   }
 
   handlePointerMove(event) {
-    // TODO(abarth): Implement event.dx;
-    var deltaX = event.x - this.previousX_;
-    this.previousX_ = event.x;
-    this.position += deltaX;
+    this.position += event.dx;
   }
 
   handlePointerUp(event) {