Don't load core-js promise into global namespace

[jason0x43]

Intern 4.3.1 is failing some Promise shim tests in @dojo/framework on FF, Safari, and Edge, but not on Node, IE, and Chrome. It appears that the core-js shim is being loaded on those platforms even though native promises are available, and it's either behaving incorrectly or causing issues with the @dojo shim.

Have Intern import Promise from core-js rather than using the shim, or use a better shim (promise-polyfill doesn't appear to have the same issue).

[jason0x43]

For reference, apparently the issue is that core-js is particularly aggressive about when to apply its polyfill, overriding native Promise implementations when this may not be desirable (zloirock/core-js#283).

[jason0x43]

promise-polyfill apparently isn't a great choice either -- it polyfills finally separately from it's standard Promise polyfill and can cause timing issues (such as finally resolving asynchronously for a canceled Task).

[gitgrimbo]

FYI @jason0x43 - I had some problems with es6-promise and polyfilling for Edge. Not sure if this impacts your work.

stefanpenner/es6-promise#330 (comment)

and

https://stackoverflow.com/questions/50096031/typeerror-object-doesnt-support-property-or-method-finally

[jason0x43]

I noticed some issues with es6-promise as well. All of these promise polyfills seem to have subtle but annoying implementation differences.

[jason0x43]

Actually, it seems the latest issue isn't so much with es6-promise as it is with some weird native Promise behavior in older versions of Firefox (< 35). Intern's polyfilling just needs to install a basically functional Promise implementation, so the issue with es6-promise not polyfilling finally shouldn't be a problem. Really, though, Intern would be better off skipping the whole polyfill business; it should import a Promise for itself (that could use the native version or a shim) and not pollute the global namespace.

[jason0x43]

Unfortunately that might end up being a significant change for downstream users, so...polyfilling for now.