[es6] Introduce spec compliant IsConstructor.

src/accessors.cc

Index: src/accessors.cc
diff --git a/src/accessors.cc b/src/accessors.cc
index 441ff4d2f54179860b90c1e8128887a62f3a35ac..c04b26681ab5142de4326888ed175e80f6277f4d 100644
--- a/src/accessors.cc
+++ b/src/accessors.cc
@@ -929,7 +929,7 @@ MUST_USE_RESULT static MaybeHandle<Object> SetFunctionPrototype(
 
 MaybeHandle<Object> Accessors::FunctionSetPrototype(Handle<JSFunction> function,
                                                     Handle<Object> prototype) {
-  DCHECK(function->should_have_prototype());
+  DCHECK(function->IsConstructor());
   Isolate* isolate = function->GetIsolate();
   return SetFunctionPrototype(isolate, function, prototype);
 }