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

src/json.js

Index: src/json.js
diff --git a/src/json.js b/src/json.js
index 8d046ee95536d1c47164afd10bf2871094454589..b66e18367e9bf2c390801ca4e19ae18c8ef9d5f3 100644
--- a/src/json.js
+++ b/src/json.js
@@ -57,7 +57,7 @@ function Revive(holder, name, reviver) {
 
 function JSONParse(text, reviver) {
   var unfiltered = %ParseJson(TO_STRING_INLINE(text));
-  if (IS_SPEC_FUNCTION(reviver)) {
+  if (IS_CALLABLE(reviver)) {
     return Revive({'': unfiltered}, '', reviver);
   } else {
     return unfiltered;
@@ -145,11 +145,11 @@ function JSONSerialize(key, holder, replacer, stack, indent, gap) {
   var value = holder[key];
   if (IS_SPEC_OBJECT(value)) {
     var toJSON = value.toJSON;
-    if (IS_SPEC_FUNCTION(toJSON)) {
+    if (IS_CALLABLE(toJSON)) {
       value = %_CallFunction(value, key, toJSON);
     }
   }
-  if (IS_SPEC_FUNCTION(replacer)) {
+  if (IS_CALLABLE(replacer)) {
     value = %_CallFunction(holder, key, value, replacer);
   }
   if (IS_STRING(value)) {