Fix spinning-square to spin again.

sky/examples/spinning-square.sky

 #!mojo mojo:sky_viewer
 <sky>
 <style>
 square {
   margin: 50px;
   height: 100px;
   width: 100px;
   background-color: green;
 }
 </style>
 <square />
 <script>
 import "dart:sky";
 
 void main() {
   Element square = document.querySelector('square');
   double timeBase = null;
 
   void animate(double time) {
     if (timeBase == null)
       timeBase = time;
     double delta = time - timeBase;
     int rotation = (delta / 10).floor();
-    square.style.setProperty("transform", 'rotate(${rotation}deg)');
+    square.style["transform"] = "rotate(${rotation}deg)";
     window.requestAnimationFrame(animate);
   }
 
   window.requestAnimationFrame(animate);
 }
 </script>
 </sky>