Implement IsConstructor() abstract operation

src/harmony-array.js

 // Copyright 2013 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.
 
 'use strict';
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 131 matching lines @@
     }
   }
 
   var iterable = ToIterable(items);
   var k;
   var result;
   var mappedValue;
   var nextValue;
 
   if (!IS_UNDEFINED(iterable)) {
-    result = IS_SPEC_FUNCTION(this) && this.prototype ? new this() : [];
+    result = %IsConstructor(this) ? new this() : [];
 
     k = 0;
     for (nextValue of items) {
       if (mapping) mappedValue = %_CallFunction(receiver, nextValue, k, mapfn);
       else mappedValue = nextValue;
       %AddElement(result, k++, mappedValue, NONE);
     }
 
     result.length = k;
     return result;
   } else {
     var len = ToLength(items.length);
-    result = IS_SPEC_FUNCTION(this) && this.prototype ? new this(len) :
-        new $Array(len);
+    result = %IsConstructor(this) ? new this(len) : new $Array(len);
 
     for (k = 0; k < len; ++k) {
       nextValue = items[k];
       if (mapping) mappedValue = %_CallFunction(receiver, nextValue, k, mapfn);
       else mappedValue = nextValue;
       %AddElement(result, k, mappedValue, NONE);
     }
 
     result.length = k;
     return result;
   }
 }
 
 // ES6, draft 05-22-14, section 22.1.2.3
 function ArrayOf() {
   var length = %_ArgumentsLength();
   var constructor = this;
   // TODO: Implement IsConstructor (ES6 section 7.2.5)
-  var array = IS_SPEC_FUNCTION(constructor) ? new constructor(length) : [];
+  var array = %IsConstructor(constructor) ? new constructor(length) : [];
   for (var i = 0; i < length; i++) {
     %AddElement(array, i, %_Arguments(i), NONE);
   }
   array.length = length;
   return array;
 }
 
 // -------------------------------------------------------------------
 
 function HarmonyArrayExtendSymbolPrototype() {
@@ skipping 20 matching lines @@
 
   // Set up the non-enumerable functions on the Array prototype object.
   InstallFunctions($Array.prototype, DONT_ENUM, $Array(
     "find", ArrayFind,
     "findIndex", ArrayFindIndex,
     "fill", ArrayFill
   ));
 }
 
 HarmonyArrayExtendArrayPrototype();