Emotion: Make jest-utils able to take in any `sheet` instance.

[tkh44]

If we can implement this feature, users of emotion will be able to use these utils as well.

expect.addSnapshotSerializer(serializer)
expect.extend(matcher)

expect.addSnapshotSerializer(serializer(sheet))
expect.extend(matcher)

This is the reason I had to fork https://github.com/kentcdodds/jest-glamor-react for the utils.

See my issue in that repo https://github.com/kentcdodds/jest-glamor-react/issues/5. I couldn't figure out how to do this without changing the api. We do not have that restriction in the repo so it can be accomplished in whatever way that works.

[mitchellhamilton]

I think this is actually possible without breaking the api.

https://github.com/mitchellhamilton/jest-glamor-react/blob/master/src/serializer.js

I'll probably open a PR once I write tests and fix it up a bit.

[kentcdodds]

That's been merged and will be released in a minute. Enjoy!

[tkh44]

@kentcdodds thank you for keeping us updated.

[tkh44]

I will port the tests over after #23 is merged.