Fix touch-demo after recent api changes.

sky/examples/touch-demo.sky

 #!mojo mojo:sky_viewer
 <sky>
 <style>
 dot {
     position: absolute;
     height: 100px;
     width: 100px;
     background-color: #00FF00;
 }
 </style>
 <dot />
 <log>Ready</log>
 <script>
 import "dart:sky";
 
 final Element dot = document.querySelector("dot");
 
 void moveDot(event) {
   double x = event.x;
   double y = event.y;
 
-  dot.style.setProperty("transform", "translate(${x-50}px,${y-50}px)");
+  dot.style["transform"] = "translate(${x-50}px,${y-50}px)";
 }
 
 void main() {
   document.addEventListener("pointerdown", moveDot);
   document.addEventListener("pointermove", moveDot);
   document.addEventListener("pointerup", moveDot);
   document.addEventListener("pointercancel", moveDot);
 }
 </script>
 </sky>