Install polyfill if clearImmediate is undefined

[robwalch]

At JW Player we've found sites that have setImmediate polyfilled without clearImmediate. This breaks our player's embed because it's compiled with webpack which depends on this module as a sub-dependency.

Checking that both setImmediate and clearImmediate are defined before exiting ensures that the polyfill is always attached when needed.

[robwalch]

@domenic are you maintaining this project? If there's anything I can do to contribute towards getting a patch please let me know. Thanks!

[robwalch]

On a side note, we'll be resolving the issue in our library by configuring webpack to not polyfill node modules (webpack node: false). promise-polyfill attempts to use setImmediate if defined, but that prompts webpack to polyfill setImmediate.

[robwalch]

ping