Sky: Add a CustomDart attribute to the IDL compiler, and use that with Canvas

sky/examples/raw/painting.sky

 <sky>
 <style>
 div {
     height: 200px;
     background-color: lightblue;
 }
 </style>
 <div id="canvas" />
 <script>
 import 'dart:math' as math;
 import 'dart:sky';
 
 void main() {
   var element = document.getElementById('canvas');
   element.requestPaint((PaintingContext context) {
     Paint paint = new Paint();
     double radius = math.min(context.width, context.height) / 2.0;
 
     context.save();
 
-    context.clipRect([0.0, 0.0, context.width, radius]);
+    context.clipRect(new Rect()..setLTRB(0.0, 0.0, context.width, radius));
 
     context.translate(context.width / 2.0, context.height / 2.0);
     paint.setARGB(128, 255, 0, 255);
     context.rotateDegrees(45.0);
-    context.drawRect([-radius, -radius, radius, radius], paint);
+
+    context.drawRect(new Rect()..setLTRB(-radius, -radius, radius, radius),
+                     paint);
 
     // Scale x and y by 0.5.
     var scaleMatrix = [
         0.5, 0.0, 0.0,
         0.0, 0.5, 0.0,
         0.0, 0.0, 1.0
     ];
     context.concat(scaleMatrix);
     paint.setARGB(128, 0, 255, 0);
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.restore();
 
     context.drawCircle(0.0, 0.0, radius, paint);
 
     context.commit();
   });
 }
 </script>
 </sky>