es5 build should not replace existing window.Promise

[jjfrankovich]

Describe the bug
My project uses a custom Promise implementation called bluebird:
https://github.com/petkaantonov/bluebird/blob/master/API.md

When media-stream-library.min.js runs (which was compiled from the master branch and built for es5) the window.Promise switches from our desired blueJay implementation to a different promise implement that is missing a few functions like Promise.finally.

Here is an example stack trace that we see after media-stream-library.min.js is loaded:

media-stream-library.min.js?version=1574257754418:8 Unhandled promise rejection TypeError: f.then(...).finally is not a function

Ideally, media-stream-library would check for the existence of window.Promise before replacing it.
Something like "window.Promise = window.Promise || media-stream-library.Promise;"

Environment:

• OS: Windows
• Browser Chrome
• Version 6.0.3 built from master branch on 20191112 for es5; we needed the ios 12 fix and no npm version has been provided with that fix yet.

[steabert]

I've read about this issue and it seems to be a general problem with combining core-js and bluebird. There seems to be a solution with zloirock/core-js#283 (https://github.com/zloirock/core-js#configurable-level-of-aggressiveness), but that would require you to rebuild the library with those options.

I'm hesitant to support this out-of-the-box, since I prefer to rely on core-js to do its job instead of making assumptions about someone's use of polyfills. They should only replace the Promise polyfill if it doesn't meet the requirements (so I guess that it's too strict for bluebird). If you can configure core-js with the less aggressive option we could add this to our documentation.

Another options would be to load bluebird after our minified bundle, if that's possible?