fix(gatsby): update check for default exports

[sidharthachatterjee]

    return _react["default"].createElement(_react["default"].Fragment, null);
  };

  return AppShell;
}(_react["default"].Component);

var _default = AppShell;
exports["default"] = _default;

The code above throws with

error #11328 

A page component must export a React component for it to be valid. Please make sure this file exports a React component:

/root/project/e2e-tests/production-runtime/node_modules/gatsby-plugin-offline/app-shell.js

This happens because our check doesn't handle exports["default"]

(babel prior to #16929 output exports.default = _default; but now outputs exports["default"] = _default;)

This PR adds that

[wardpeet]

Nice catch @sidharthachatterjee any chance we can write an unit test or e2e test to catch this issue? Let's do it in a follow up. I'm happy to do so as well.

[sidharthachatterjee]

@wardpeet Will be painful to write as an end to end because we'll need different versions of babel for the different outputs 😓

But we can add fixtures for each compiled case and write some unit tests I think. What do you think?

[wardpeet]

I didn't really look at the whole file so it was more a go to responds :p So not worth at this point to actually add a test. I also think we have covered all possible options so 👍

[sidharthachatterjee]

@wardpeet Yeah, I agree. Let's table this for now. Perhaps in the future, we can improve the heuristic that it uses to check whether there is a default export at all. For now, we're good!

Thanks! 💜