Make it possible to custom-paint without an Element.

sky/examples/display_list/full_screen_circle.dart

+import "dart:sky";
+import "dart:math";
+
+void doIt() {
+  double width = window.innerWidth.toDouble();
+  double height = window.innerHeight.toDouble();
+
+  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);
+
+  document.rootPicture = recorder.endRecording();
+}