Add wheel support to sky-scrollable

sky/framework/sky-scrollable.sky

Index: sky/framework/sky-scrollable.sky
diff --git a/sky/framework/sky-scrollable.sky b/sky/framework/sky-scrollable.sky
index 5adc6841d3e758adc15573abc3b74f5d28eb4517..33fe82822c33e712a741978d58e8f017c9a76c22 100644
--- a/sky/framework/sky-scrollable.sky
+++ b/sky/framework/sky-scrollable.sky
@@ -12,7 +12,8 @@
   on-gesturescrollend="handleScrollEnd_"
   on-gesturescrollupdate="handleScrollUpdate_"
   on-gestureflingstart="handleFlingStart_"
-  on-gestureflingcancel="handleFlingCancel_">
+  on-gestureflingcancel="handleFlingCancel_"
+  on-wheel="handleWheel_">
 <template>
   <style>
   :host {
@@ -115,6 +116,10 @@ module.exports = class extends SkyElement {
   handleFlingCancel_(event) {
     this.stopFling_();
   }
+
+  handleWheel_(event) {
+    this.scrollBy(-event.offsetY);
+  }
 }.register();
 </script>
 </sky-element>