[promise] Test IsPromise() early in Promise.prototype.then()

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();
 
@@ skipping 256 matching lines @@
   } else {
     var promiseCapability = NewPromiseCapability(this);
     %_Call(promiseCapability.reject, UNDEFINED, r);
     return promiseCapability.promise;
   }
 }
 
 // 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;
   onResolve = IS_CALLABLE(onResolve) ? onResolve : PromiseIdResolveHandler;
   onReject = IS_CALLABLE(onReject) ? onReject : PromiseIdRejectHandler;
   var deferred = NewPromiseCapability(constructor);
-  switch (GET_PRIVATE(this, promiseStatusSymbol)) {
-    case UNDEFINED:
-      // TODO(littledan): The type check should be called before
-      // constructing NewPromiseCapability; this is observable when
-      // erroneously copying this method to other classes.
-      throw MakeTypeError(kNotAPromise, this);
+  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),
                      [onResolve, deferred],
                      +1);
       break;
     case -1:  // Rejected
@@ skipping 158 matching lines @@
 [PromiseChain, PromiseDeferred, PromiseResolved].forEach(
     fn => %FunctionRemovePrototype(fn));
 
 utils.Export(function(to) {
   to.PromiseChain = PromiseChain;
   to.PromiseDeferred = PromiseDeferred;
   to.PromiseResolved = PromiseResolved;
 });
 
 })