Align GetIterator with ES6 spec

src/weak-collection.js

Index: src/weak-collection.js
diff --git a/src/weak-collection.js b/src/weak-collection.js
index a44c3d7cd7caeb2d7eb835b002332fc8df30a679..3cec9cc5071c6add9c3b9cd1830abaf82c859bbe 100644
--- a/src/weak-collection.js
+++ b/src/weak-collection.js
@@ -23,7 +23,7 @@ function WeakMapConstructor(iterable) {
   var iter, adder;
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
-    iter = GetIterator(ToObject(iterable));
+    iter = GetIterator(iterable);
     adder = this.set;
     if (!IS_SPEC_FUNCTION(adder)) {
       throw MakeTypeError('property_not_function', ['set', this]);
@@ -130,7 +130,7 @@ function WeakSetConstructor(iterable) {
   var iter, adder;
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
-    iter = GetIterator(ToObject(iterable));
+    iter = GetIterator(iterable);
     adder = this.add;
     if (!IS_SPEC_FUNCTION(adder)) {
       throw MakeTypeError('property_not_function', ['add', this]);