don't mark unparsed .js.flow files as checked
don't mark unparsed .js.flow files as checked

Summary:
parsed `.js.flow` files are not automatically considered "checked", so don't consider unparsed `.js.flow` files as such either.

as far as I can tell, this doesn't really make a difference since we only check parsed files, but I want to kill `Docblock.isDeclarationFile`.

Reviewed By: samwgoldman

Differential Revision: D20069817

fbshipit-source-id: ae9d654c0f6bd47fbc18eef5ee3deb1cb132328c

facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow