Avoid babel polyfill collision when loaded multiple times by differen…

[apwidejulien]

Babel-polyfill is embedded in the published version of timeline-plus, which cause trouble when integrating with other libraries using this polyfill too.

In fact, I'm a bit surprised of the issue, because it looks that babel polyfill is added as devDependencies in your project (strange regarding documentation which specify that it should be added as a dependency https://babeljs.io/docs/en/babel-polyfill) but despite this, the polyfill is still part of the published version.

The issue with babel-polyfill (I think it depends on the version of babel), is that if you add it twice, it will raise an error instead of just a warning. A trick has been added to babel (babel/babel#6371) for muting the double import (which is probably a better approach when using polyfill in a library).

[yotamberk]

Thanks for the correction and PR. I will look at the docs and see why this happened in the first place