Make tests/clipping/canvas-rounded-corners.sky actually run.

sky/tests/clipping/canvas-rounded-corners.sky

 <sky>
   <import src="../resources/dump-as-render-tree.sky" />
   <style>
     canvas {
       background-color: gray;
       border: 10px solid red;
       border-radius: 200px 100px 0px 0px;
     }
   </style>
   <!-- TODO(ojan): Make this a reftest. -->
   <h3>It passes if:</h3>
   <ul>
     <li>the canvas content has rounded corners (top-left and top-right)</li>
     <li>the 10px red border is visible around the canvas content</li>
     <li>gray border is not visible</li>
   </ul>
   <canvas id="theCanvas" width="257" height="257"></canvas>
   <script>
+  import 'dart:sky';
+
   void main() {
     var theCanvas = document.getElementById('theCanvas');
     var context = theCanvas.getContext('2d');
     context.fillColor = 'green';
-    context.fillRect(0, 0, theCanvas.width, theCanvas.width);
+    context.fillRect(0.0, 0.0, 1.0 * theCanvas.width, 1.0 * theCanvas.width);
   }
   </script>
 </sky>