Teach SkyView path to draw a circle

This CL adds a global view object that can receive a Picture and be signaled to
draw. When using SkyView, this Picture shows up on screen.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1134913003

examples/raw/hello_world.dart

@@ -2,6 +2,25 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import "dart:math";
+import 'dart:sky';
+
 void main() {
-    print("Hello, world");
+  print("Hello, world");
+
+  double width = 500.0;
+  double height = 500.0;
+
+  PictureRecorder recorder = new PictureRecorder(width, height);
+  double radius = min(width, height) * 0.45;
+
+  Paint paint = new Paint()..setARGB(255, 0, 255, 0);
+
+  recorder.drawCircle(width / 2, height / 2, radius, paint);
+
+  print("Storing picture");
+  view.picture = recorder.endRecording();
+
+  print("Scheduling paint");
+  view.schedulePaint();
 }