Ship additional required polyfills

[taion]

Follow-up from #198, #238.

Per http://babeljs.io/docs/usage/caveats/#polyfills, generators are not the only supported language feature that require a polyfill.

Array destructuring, for-of loops, and the array spread operator all require additional polyfills. As such, this project should also ship polyfills for Symbol and Array.from.

It would be possible to accomplish when needed this just by re-enabling transpiling the helpers for the Babel runtime transform, though because of https://phabricator.babeljs.io/T7519, a few of the helpers pull in the Symbol polyfill unnecessarily.

[insin]

Is it potentially problematic that babel-runtime vendors core-js? If react-scripts takes a dependency on core-js for default polyfills, it won't be deduplicated by npm.

https://npmcdn.com/babel-runtime/

[taion]

TIL. Wow, that's a bit inconvenient for this use case. I guess babel-runtime has to vendor core-js though.

I suggested babel-runtime as it would only bring in those polyfills if needed. Directly bringing in the polyfills would also work, though.

[insin]

Having Symbol guaranteed would also mean the inline elements transform could be used for production builds.

[gaearon]

For future reference, this is currently blocked on babel/babel#4418.

[clarkbw]

The babel issue babel/babel#4418 is closed now. However I think this issue could be closed as well because I believe the rest is covered by #2391 (Babel 7 Umbrella)

[taion]

Thanks @clarkbw! I think Inbox swallowed my email notification here or something, so I didn't see this until just now.