ES6 collections: Fix order of constructor logic

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index 9b5716687bcc14ed06e628afd15de795b07c1a14..8d2d18428602b85979ff98afde6f60ea2c30ff92 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -23,11 +23,11 @@ function SetConstructor(iterable) {
   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);
@@ -163,11 +163,11 @@ function MapConstructor(iterable) {
   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);