Summary:
Virtual "this" invisible param exists in the annotated signature, but
does not exist in some other places, which causes a lot of annoyance in
different places.
This diff does not intend to solve all this, but makes one step forward.
1/ AnnotatedNullability now explicitly distincts normal params and
VirtualThis.
2/ AnnotatedSignature accounts for this via: a) not having redundant
fake annotation points b) having corrent param indices (those were off
by 1 in non-virtul methods).
Reviewed By: artempyanykh
Differential Revision: D24726480
fbshipit-source-id: fdb8bb0fb
Mitya Lyubarskiy