ES6 collections: Fix order of constructor logic

src/collection.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.
 
 "use strict";
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Array = global.Array;
 
 var $Set = global.Set;
 var $Map = global.Map;
 
 
 // -------------------------------------------------------------------
 // Harmony Set
 
 function SetConstructor(iterable) {
   if (!%_IsConstructCall()) {
     throw MakeTypeError('constructor_not_function', ['Set']);
   }
 
   var iter, adder;
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
-    iter = GetIterator(iterable);
     adder = this.add;
     if (!IS_SPEC_FUNCTION(adder)) {
       throw MakeTypeError('property_not_function', ['add', this]);
     }
+    iter = GetIterator(iterable);
   }
 
   %_SetInitialize(this);
 
   if (IS_UNDEFINED(iter)) return;
 
   var next, done;
   while (!(next = iter.next()).done) {
     if (!IS_SPEC_OBJECT(next)) {
       throw MakeTypeError('iterator_result_not_an_object', [next]);
@@ skipping 115 matching lines @@
 // Harmony Map
 
 function MapConstructor(iterable) {
   if (!%_IsConstructCall()) {
     throw MakeTypeError('constructor_not_function', ['Map']);
   }
 
   var iter, adder;
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
-    iter = GetIterator(iterable);
     adder = this.set;
     if (!IS_SPEC_FUNCTION(adder)) {
       throw MakeTypeError('property_not_function', ['set', this]);
     }
+    iter = GetIterator(iterable);
   }
 
   %_MapInitialize(this);
 
   if (IS_UNDEFINED(iter)) return;
 
   var next, done, nextItem;
   while (!(next = iter.next()).done) {
     if (!IS_SPEC_OBJECT(next)) {
       throw MakeTypeError('iterator_result_not_an_object', [next]);
@@ skipping 114 matching lines @@
     "get", MapGetJS,
     "set", MapSetJS,
     "has", MapHasJS,
     "delete", MapDeleteJS,
     "clear", MapClearJS,
     "forEach", MapForEach
   ));
 }
 
 SetUpMap();