sourcemaps break the css`` API

[johnbrett]

Fantastic work on this library btw, I was last working with v6, lots of improvements since I see 👍

• emotion version: 8.0.10
• react version: 16.1.0

Relevant code.

const redBG = css`background-color: red`
injectGlobal`
  h2 {
    color: green;
  }
`

class App extends Component {
  render() {
    return (
      <div className={redBG}>
          <h2>testing</h2>
      </div>
    );
  }
}

What you did:
I enabled sourceMaps in a create-react-app rewired project:

module.exports = function override(config, env) {
  return rewireEmotion(config, env, { sourceMap: true  });
}

What happened:
The effects of the css API no longer worked, although the css class was created on the element.

Reproduction:

Github repo: https://github.com/johnbrett/emotion-testing

Happy to try debug this further / submit a fix with guidance!