IE11: Update babel config to use the runtime

[oigewan]

Bug:
Fixes #403

As mentioned in the issue, this library breaks in IE11 due to missing polyfills. You could just document the polyfill requirements for this library, but since IE 11 is still over 2% of browser usage and this is a library, I think it'd be a good idea to set up babel to use polyfills and helpers.

I also had issues testing changes to the library. When I ran npm link to test my changes, I kept getting errors saying that webpack could not find the "react" library. I looked and saw that React is listed as a peerDependency. However, since React is imported by the library's code and is required in order for this library to compile properly, I'm thinking it should be a dependency.

Notes:
Babel documentation on the plugins I'm using and dependencies I added.

Changes:

• Add the transform-runtime plugin configured to use the helpers and polyfills.
• Add a dependency on the babel-runtime.
• Moved React from a peerDependency to a dependency.

cc: @gpbl

[gpbl]

Thanks a lot for helping with this! I've added some comments to your changes.

[gpbl]

Any reason why you are removing react from peerDependencies?

[oigewan]

Mentioned in the PR description, but I'll copy/paste here:

I also had issues testing changes to the library. When I ran npm link to test my changes, I kept getting errors saying that webpack could not find the "react" library. I looked and saw that React is listed as a peerDependency. However, since React is imported by the library's code and is required in order for this library to compile properly, I'm thinking it should be a dependency.

[gpbl]

Sorry I missed that part 😄

React is included in the devDependencies, which should be installed when developing the module. So definitely react must stay as peerDependency here.

[gpbl]

Do you know the impact this is having to the final bundle size? I'd rather remove the stuff not supported by IE11.

[oigewan]

The bundle size did go up a noticeable amount. I did some investigation on what would need to be done. The Array.from references are actually introduced by a babel plugin, so it's affects aren't quite easy to see.

I'll close this PR and open another one as soon as I'm capable of testing my changes.

[gpbl]

@oigewan

• These lines in Day.js could be use the object-assign module (to add as dependency).
• I'm still wondering where does this Array.from come from 🤔

[gpbl]

@oigewan ah the Array.from comes from spreading arrays, e.g. here. We can also fix that without adding other dependencies...

[oigewan]

Yup. New PR: #406