Revert "PromiseThen should ignore non-function parameters."

src/promise.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 197 matching lines @@
     }
     if (uncaught) %DebugPendingExceptionInPromise(exception, deferred.promise);
   }
 }
 
 
 // Multi-unwrapped chaining with thenable coercion.
 
 function PromiseThen(onResolve, onReject) {
   onResolve =
-    IS_SPEC_FUNCTION(onResolve) ? onResolve : PromiseIdResolveHandler;
+    IS_NULL_OR_UNDEFINED(onResolve) ? PromiseIdResolveHandler : onResolve;
   onReject =
-    IS_SPEC_FUNCTION(onReject) ? onReject : PromiseIdRejectHandler;
+    IS_NULL_OR_UNDEFINED(onReject) ? PromiseIdRejectHandler : onReject;
   var that = this;
   var constructor = this.constructor;
   return %_CallFunction(
     this,
     function(x) {
       x = PromiseCoerce(constructor, x);
       return x === that ? onReject(MakeTypeError('promise_cyclic', [x])) :
              IsPromise(x) ? x.then(onResolve, onReject) : onResolve(x);
     },
     onReject,
@@ skipping 117 matching lines @@
   return GET_PRIVATE(promise, promiseOnResolve);
 }
 
 function GetPromiseOnReject(promise) {
   return GET_PRIVATE(promise, promiseOnReject);
 }
 
 function GetPromiseValue(promise) {
   return GET_PRIVATE(promise, promiseValue);
 }