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

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 8e7fc6c01c6889a8a5ccecf399ef7a538bafdac4..c20c7372fdca63d8daacf2859717411fdcc77d9d 100644
--- a/src/string.js
+++ b/src/string.js
@@ -248,7 +248,7 @@ function StringReplace(search, replace) {
     var lastIndex = search.lastIndex;
     TO_INTEGER_FOR_SIDE_EFFECT(lastIndex);
 
-    if (!IS_SPEC_FUNCTION(replace)) {
+    if (!IS_CALLABLE(replace)) {
       replace = TO_STRING_INLINE(replace);
 
       if (!search.global) {
@@ -315,7 +315,7 @@ function StringReplace(search, replace) {
   var result = %_SubString(subject, 0, start);
 
   // Compute the string to replace with.
-  if (IS_SPEC_FUNCTION(replace)) {
+  if (IS_CALLABLE(replace)) {
     var receiver = UNDEFINED;
     result += %_CallFunction(receiver, search, start, subject, replace);
   } else {