fix: only load Promise polyfill if window.Promise is missing

[straker]

es6-promises doesn't check if window.Promise exists before overriding it, but instead checks if the browser natively supports Promises by seeing if Promise.resolve() returns '[object Promise]' when passed to toString:

let P = local.Promise;

if (P) {
  var promiseToString = null;
  try {
    promiseToString = Object.prototype.toString.call(P.resolve());
  } catch(e) {
    // silently ignored
  }

  if (promiseToString === '[object Promise]' && !P.cast){
    return;
  }
}

local.Promise = Promise;

Adding a check before we run the polyfill to see if window.Promise exists.

I looked at changing to anther promise library that would do the check better, but I'm not sure if @JKODU chose es6-promsie for a specific reason or not. promise-polyfill claims it's <1 kb when gzipped, which appears to be much smaller than es6-promise (2.4 KB gzipped), and it looks to see if window.Promise exists before polyfilling.

Closes: #1468

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