Avoid loading Promise polyfill in modern browsers.

[lgarron]

It's only needed on IE. :-/

Instead of pretty native Promise, I see the polyfill even in Chrome right now.

[the-spyke]

Just bumped into this issue when I was trying to debug .writeText(...).finally is not a function error.

I work in a browser with no Promise.prototype.finally(), but I do use babel-polyfill which has no power in this case :-(

[lgarron]

Yeah, I wish there were a simple solution. Open to (compatible) suggestions.

[the-spyke]

I'd suggest to build without polyfills and let users use their own if they need it. Just add to readme what ES2015+ features this library requires.

[lgarron]

The main point of this library is to be as compatible as possible with old browsers while the new API is rolling out. I don't feel comfortable removing compatibility with IE by default, although I'm okay with users opting out of the polyfill.

[lgarron]

99de811 is not super-clean, but it passes the type checks and avoids using es6-promise in browser that already have Promise.

I would prefer not to support multiple builds unless there are specific concerns about build size, so hopefully this will do for now.