Add a basic custom painting facility to Sky

This CL adds just enough custom painting to Sky to make
sky/examples/painting/circle.sky draw a circle. Over time, we should be able to
elaborate this system into something interesting and to make it actually work
in a reasonable way.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/1017593005

examples/painting/circle.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();
+    paint.setARGB(128, 0, 255, 0);
+    double radius = math.min(context.width, context.height) / 2.0;
+    context.drawCircle(context.width / 2.0, context.height / 2.0, radius, paint);
+    context.commit();
+  });
+}
+</script>
+</sky>