Summary: Mirrors Task. One behavioral difference is that yielding an empty Try from a Task converts into a UsingUninitializedTry exception, while for AsyncGenerator it converts into a return. This is for consistency with Thrift's ClientBufferedStream to avoid confusing users, but there is also an argument for converting to UsingUninitializedTry instead.

Reviewed By: yfeldblum

Differential Revision: D25379361

fbshipit-source-id: 2a2878185696566b29345c99fee7ba74bef4ae8a