Thanks for the suggestions everyone. In the end, I decided to extend Quad and then edit starling.display.Image to extend that instead of the original Quad. I just have to remember to edit Image every time I update Starling.
If you're wondering, I was trying to add a color setter for the Sprite class. At first I thought it would be easy, and all I had to do was add a method that would set each child Mesh's color, but then I realized, what if the child already had a color? That color would have to be multiplied. So I created an InterfaceRGB interface, which my DisplayObjectContainerRGB and QuadRGB classes implement. Sprite, Sprite3D, and TextField now extend DisplayObjectContainerRGB, and Image extends QuadRGB. Basically, my RGB classes have a "true" RGB vector and a "multiplied" RGB vector. When a child gets added, the child's multiplied vector is its true vector multiplied by the parent's multiplied vector, then the child's multiplied vector is used to set it's displayed color.