Summary:
`declareOpts` prevents strong Flow typing, and promotes default values, that may be different from a function to another (my goal is to remove defaults from the whole codebase except perhaps the public API). This changeset replaces it by Flow types and fixes callsites to be explicit on values.
This is the last callsite of `declareOpts` so I'll remove it, in a separate diff.
Reviewed By: cpojer
Differential Revision: D4970650
fbshipit-source-id: e5ea2e06febde892d28c9dc59dc2920d4033bb01