Fix sinon flow bindings

[yamadapc]

Fixes flow type bindings so that further refinement assertions can be built
with typechecking.

For example, this is currently failing to typecheck:

const writeFileSync = sinon.stub(fs, 'writeFileSync');
const count = writeFileSync.withArgs('f', 'contents').callCount;

And will typecheck now.

Test Plan: yarn flow