Not supported in 4.4, in 4.5 you’ll be able to give it a negative scale in the render transform options to flip them. Using the handles to do it will likely not work by 4.5.
Rebirthing this in order to point the difference between flip the image sprite and the scale. As in Unity
For example. If you set a negative scale to a button, their attached elements will be negative-scaled too, like a usual text. However, having a flip option avoid the children of a flipped (X or Y) sprite to become negative-scaled.