Promise prototype is being erased

[RodionM]

We are using "finally" Promise polifill since this function is not supported by Internet Explorer browser.
After integration of FileManager it turned out that function "finally" disappeared from Promise prototype.

After brief check of FileManager package code I have found following piece of code:

// constructor polyfill
if (!USE_NATIVE) {
  // 25.4.3.1 Promise(executor)
  $Promise = function Promise(executor) {
    anInstance(this, $Promise, PROMISE, '_h');
    aFunction(executor);
    Internal.call(this);
    try {
      executor(ctx($resolve, this, 1), ctx($reject, this, 1));
    } catch (err) {
      $reject.call(this, err);
    }
  };
  // eslint-disable-next-line no-unused-vars
  Internal = function Promise(executor) {
    this._c = [];             // <- awaiting reactions
    this._a = undefined;      // <- checked in isUnhandled reactions
    this._s = 0;              // <- state
    this._d = false;          // <- done
    this._v = undefined;      // <- value
    this._h = 0;              // <- rejection state, 0 - default, 1 - handled, 2 - unhandled
    this._n = false;          // <- notify
  };
  Internal.prototype = __webpack_require__(303)($Promise.prototype, {
    // 25.4.5.3 Promise.prototype.then(onFulfilled, onRejected)
    then: function then(onFulfilled, onRejected) {
      var reaction = newPromiseCapability(speciesConstructor(this, $Promise));
      reaction.ok = typeof onFulfilled == 'function' ? onFulfilled : true;
      reaction.fail = typeof onRejected == 'function' && onRejected;
      reaction.domain = isNode ? process.domain : undefined;
      this._c.push(reaction);
      if (this._a) this._a.push(reaction);
      if (this._s) notify(this, false);
      return reaction.promise;
    },
    // 25.4.5.1 Promise.prototype.catch(onRejected)
    'catch': function (onRejected) {
      return this.then(undefined, onRejected);
    }
  });
  OwnPromiseCapability = function () {
    var promise = new Internal();
    this.promise = promise;
    this.resolve = ctx($resolve, promise, 1);
    this.reject = ctx($reject, promise, 1);
  };
  newPromiseCapabilityModule.f = newPromiseCapability = function (C) {
    return sameConstructor($Promise, C)
      ? new OwnPromiseCapability(C)
      : newGenericPromiseCapability(C);
  };
}

It looks like Promise prototype is being erased and only "then" and "catch" functions are being added.