code: Image fillImage() {
      if (fillImage == null) {
        CanvasImage image = platform.graphics().createImage(1, 1);
        image.canvas().setFillColor(0xFFFFFFFF).fillRect(0, 0, image.width(), image.height());
        fillImage = image;
      }
      return fillImage;
    }
docs: Returns a (created on demand, then cached) image used when filling solid color quads or
  triangles.