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

src/runtime/runtime-classes.cc

Index: src/runtime/runtime-classes.cc
diff --git a/src/runtime/runtime-classes.cc b/src/runtime/runtime-classes.cc
index 8692b9b80054cd81831cf2bfa102d420da1af452..32b00f734c74aaee9d8909b62b516ce74c90820e 100644
--- a/src/runtime/runtime-classes.cc
+++ b/src/runtime/runtime-classes.cc
@@ -104,7 +104,7 @@ static MaybeHandle<Object> DefineClass(Isolate* isolate, Handle<Object> name,
   } else {
     if (super_class->IsNull()) {
       prototype_parent = isolate->factory()->null_value();
-    } else if (super_class->IsSpecFunction()) {
+    } else if (super_class->IsJSFunction()) {

[rossberg, 2015/08/26 12:55:48]

Shouldn't this be IsCallable as well? It's perfectly legal to inherit from, say,
a function proxy.

[Benedikt Meurer, 2015/08/27 05:18:23]

Actually it should be IsConstructor, but we don't have that yet. Plus there's a
JSFunction cast on the next line. So I'll add a TODO and clean it up later once
we have proper IsConstructor.

       if (Handle<JSFunction>::cast(super_class)->shared()->is_generator()) {
         THROW_NEW_ERROR(
             isolate,