Bring back babel

[bramkragten]

This PR was open longer than expected because we ran into an issue with properties. Putting it here to make sure we don't forget.

In TypeScript we often declare empty properties to make TS aware of the type:

class Hello {
  public name: string
}

With the latest class fields proposal these should now result in the value undefined being set with a defineProperty (TS issue). This causes any Lit Element setters generated via static get propertes() to be erased.

However, we now run into the issue that we cannot use decorators inside anonymous classes (used by our mixins). So in this PR we add // @ts-ignore to the three impacted mixins.

Fixes #3830

[iantrich]

Fix for #3830?

[bramkragten]

Fix for #3830?

Yes, but it is not working yet... 😢