feature request: Add opaque type

[clark-pan]

Flow recently added the opaque type.

I've tried, but I can't seem to figure out where in the syntax to add it... I've only found 1 location but i can't seem to find the main syntax declaration for type.

[borela]

I added a couple commits on the master branch to support opaque types, please check if it's working.

[clark-pan]

Yep looking good! Thank you.

[borela]

Released the fix in v2.6.0.