Assert that NativeConstructors return object when constructed

Summary:
`NativeConstructor`s make their own `this` when constructed. This means
that we *must* be returning an object out of `NativeConstructor`s in
this situation. Add an assert for this invariant.

Reviewed By: tmikov

Differential Revision: D66511004

fbshipit-source-id: a9ed322f9197cf458e99e21f781cd9f5f59e71e1

lib/VM/Callable.cpp

@@ -1332,7 +1332,13 @@ CallResult<PseudoHandle<>> NativeConstructor::_callImpl(
       newTarget.isUndefined() ||
       vmisa<Callable>(newTarget) &&
           "new.target for a NativeConstructor should either be undefined or a callable");
-  return NativeFunction::_callImpl(selfHandle, runtime);
+  auto res = NativeFunction::_callImpl(selfHandle, runtime);
+  if (LLVM_LIKELY(res != ExecutionStatus::EXCEPTION)) {
+    assert(
+        (newTarget.isUndefined() || vmisa<JSObject>(res->getHermesValue())) &&
+        "NativeConstructor needs to return an object when called as constructor.");
+  }
+  return res;
 }
 #endif