ES6 collections: Fix order of constructor logic

src/weak-collection.js

Index: src/weak-collection.js
diff --git a/src/weak-collection.js b/src/weak-collection.js
index 3cec9cc5071c6add9c3b9cd1830abaf82c859bbe..33e3d4c69758c8c052822c98f5d0890ee7be9e81 100644
--- a/src/weak-collection.js
+++ b/src/weak-collection.js
@@ -23,11 +23,11 @@ function WeakMapConstructor(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);
   }
 
   %WeakCollectionInitialize(this);
@@ -130,11 +130,11 @@ function WeakSetConstructor(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);
   }
 
   %WeakCollectionInitialize(this);