Add @@species/better subclassing support to Promises

src/js/promise.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var InternalArray = utils.InternalArray;
 var MakeTypeError;
 var promiseCombinedDeferredSymbol =
     utils.ImportNow("promise_combined_deferred_symbol");
 var promiseHasHandlerSymbol =
     utils.ImportNow("promise_has_handler_symbol");
 var promiseOnRejectSymbol = utils.ImportNow("promise_on_reject_symbol");
 var promiseOnResolveSymbol =
     utils.ImportNow("promise_on_resolve_symbol");
 var promiseRawSymbol = utils.ImportNow("promise_raw_symbol");
 var promiseStatusSymbol = utils.ImportNow("promise_status_symbol");
 var promiseValueSymbol = utils.ImportNow("promise_value_symbol");
+var SpeciesConstructor;
 var toStringTagSymbol = utils.ImportNow("to_string_tag_symbol");
 
 utils.Import(function(from) {
   MakeTypeError = from.MakeTypeError;
+  SpeciesConstructor = from.SpeciesConstructor;
 });
 
 // -------------------------------------------------------------------
 
 // Status values: 0 = pending, +1 = resolved, -1 = rejected
 var lastMicrotaskId = 0;
 
 function CreateResolvingFunctions(promise) {
   var alreadyResolved = false;
 
@@ skipping 234 matching lines @@
 }
 
 // Multi-unwrapped chaining with thenable coercion.
 
 function PromiseThen(onResolve, onReject) {
   var status = GET_PRIVATE(this, promiseStatusSymbol);
   if (IS_UNDEFINED(status)) {
     throw MakeTypeError(kNotAPromise, this);
   }
 
-  var constructor = this.constructor;
+  var constructor = SpeciesConstructor(this, GlobalPromise);
   onResolve = IS_CALLABLE(onResolve) ? onResolve : PromiseIdResolveHandler;
   onReject = IS_CALLABLE(onReject) ? onReject : PromiseIdRejectHandler;
   var deferred = NewPromiseCapability(constructor);
   switch (status) {
     case 0:  // Pending
       GET_PRIVATE(this, promiseOnResolveSymbol).push(onResolve, deferred);
       GET_PRIVATE(this, promiseOnRejectSymbol).push(onReject, deferred);
       break;
     case +1:  // Resolved
       PromiseEnqueue(GET_PRIVATE(this, promiseValueSymbol),
@@ skipping 180 matching lines @@
 [PromiseChain, PromiseDeferred, PromiseResolved].forEach(
     fn => %FunctionRemovePrototype(fn));
 
 utils.Export(function(to) {
   to.PromiseChain = PromiseChain;
   to.PromiseDeferred = PromiseDeferred;
   to.PromiseResolved = PromiseResolved;
 });
 
 })