What is the symbolism of the dark haried girl "throwing aside her uniform" in Winston's dream?

I think this represents a couple of things.

First, you ask about the symbol of throwing off the uniform. I think this symbolizes throwing off the Party from one's self. She is letting go of the exterior that she has to maintain to reveal the real self on the inside.

Second, I think this action is very literal too. Sex is frowned upon by the party, and Winston allowing his mind to go to the place to actually long for it is rare, but human. Winston is being a normal and very real man imagining a woman undressing.