That's because there exist retainAll and removeAll methods on Collection that are being chosen instead.
I did notice that content assist for list literals wasn't working for empty lists. Eg, ._ was not returning any useful proposals. There was a problem with our completion tokenizer. This is now fixed.
So, is the problem that you weren't seeing the content assist proposals, or that you want the editor to recognize the DGM method instead of the method on collection?