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

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 0dafe03e7a88658e5acb0698d21d15387274a221..889a76094611dd7b5624494f5f74d5ba442a4a74 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -36,6 +36,7 @@ void Object::VerifyPointer(Object* p) {
 
 void Smi::SmiVerify() {
   CHECK(IsSmi());
+  CHECK(!IsCallable());
 }
 
 
@@ -533,6 +534,7 @@ void JSFunction::JSFunctionVerify() {
   CHECK(next_function_link() == NULL ||
         next_function_link()->IsUndefined() ||
         next_function_link()->IsJSFunction());
+  CHECK(map()->is_callable());
 }
 
 
@@ -811,6 +813,7 @@ void JSFunctionProxy::JSFunctionProxyVerify() {
   JSProxyVerify();
   VerifyPointer(call_trap());
   VerifyPointer(construct_trap());
+  CHECK(map()->is_callable());
 }