BitmapData.fromBitmapData(BitmapData bitmapData, Rectangle<num> rectangle)

Source 

factory BitmapData.fromBitmapData(BitmapData bitmapData, Rectangle<num> rectangle) {
  var renderTextureQuad = bitmapData.renderTextureQuad.cut(rectangle);
  return new BitmapData.fromRenderTextureQuad(renderTextureQuad);
}