es6-promise/lib/es6-promise/polyfill.js

/*global self*/
import Promise from './promise';

export default function polyfill() {
  let local;

  if (typeof global !== 'undefined') {
    local = global;
  } else if (typeof self !== 'undefined') {
    local = self;
  } else {
    try {
      local = Function('return this')();
    } catch (e) {
      throw new Error('polyfill failed because global object is unavailable in this environment');
    }
  }

  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;
}