Add @@species/better subclassing support to Promises

src/js/promise.js

Index: src/js/promise.js
diff --git a/src/js/promise.js b/src/js/promise.js
index 48ed821373063185c2c288881bb3296a22facfcc..dd921ced46c0ad78d4261abdd634ca1f07b6ce99 100644
--- a/src/js/promise.js
+++ b/src/js/promise.js
@@ -23,10 +23,12 @@ var promiseOnResolveSymbol =
 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;
 });
 
 // -------------------------------------------------------------------
@@ -281,7 +283,7 @@ function PromiseThen(onResolve, onReject) {
     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);