[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 4bc61b8f6b1d4374ff5d8f85b43baf5c86f13429..8b22d05fd3d5fc23b3ea1ee06c0caddc995768d2 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -1116,7 +1116,7 @@ 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());