[es6] Introduce spec compliant IsConstructor.

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 8209774d1510a7a5e84fd3712af3e47874ef00c1..beefbaa964bd79ede95d875203cf779eda4f8288 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -22,6 +22,7 @@ void Object::ObjectVerify() {
   } else {
     HeapObject::cast(this)->HeapObjectVerify();
   }
+  CHECK(!IsConstructor() || IsCallable());
 }
 
 
@@ -37,6 +38,7 @@ void Object::VerifyPointer(Object* p) {
 void Smi::SmiVerify() {
   CHECK(IsSmi());
   CHECK(!IsCallable());
+  CHECK(!IsConstructor());
 }