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

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index 261c58317fb7fd3084dcb56b7741bb3bce282132..01a1191bb68520e7f7ec1261c8fded49fb874074 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -1109,7 +1109,7 @@ MUST_USE_RESULT static Object* HandleApiCallAsFunctionOrConstructor(
 
   // Get the invocation callback from the function descriptor that was
   // used to create the called object.
-  DCHECK(obj->map()->has_instance_call_handler());
+  DCHECK(obj->map()->is_callable());
   JSFunction* constructor = JSFunction::cast(obj->map()->GetConstructor());
   // TODO(ishell): turn this back to a DCHECK.
   CHECK(constructor->shared()->IsApiFunction());