[es6] Initial steps towards a correct implementation of IsCallable.

src/weak-collection.js

Index: src/weak-collection.js
diff --git a/src/weak-collection.js b/src/weak-collection.js
index 75350931ed22fed9577e37ef1b56756c59c1d116..bd15f862eb86c4bcf53de13daf2b54e3ccc00381 100644
--- a/src/weak-collection.js
+++ b/src/weak-collection.js
@@ -24,7 +24,7 @@ function WeakMapConstructor(iterable) {
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
     var adder = this.set;
-    if (!IS_SPEC_FUNCTION(adder)) {
+    if (!IS_CALLABLE(adder)) {
       throw MakeTypeError(kPropertyNotFunction, 'set', this);
     }
     for (var nextItem of iterable) {
@@ -113,7 +113,7 @@ function WeakSetConstructor(iterable) {
 
   if (!IS_NULL_OR_UNDEFINED(iterable)) {
     var adder = this.add;
-    if (!IS_SPEC_FUNCTION(adder)) {
+    if (!IS_CALLABLE(adder)) {
       throw MakeTypeError(kPropertyNotFunction, 'add', this);
     }
     for (var value of iterable) {