Re-definition/Assets/Plugins/Unity UI Extensions/Runtime/Scripts/Utilities/ExtentionMethods.cs

using System;

namespace UnityEngine.UI.Extensions
{
    public static class ExtentionMethods
    {
        public static T GetOrAddComponent<T>(this GameObject child) where T : Component
        {
            T result = child.GetComponent<T>();
            if (result == null)
            {
                result = child.AddComponent<T>();
            }
            return result;
        }

        public static bool IsPrefab(this GameObject gameObject)
        {
            if (gameObject == null)
            {
                throw new ArgumentNullException(nameof(gameObject));
            }

            return
                !gameObject.scene.IsValid() &&
                !gameObject.scene.isLoaded &&
                gameObject.GetInstanceID() >= 0 &&
                // I noticed that ones with IDs under 0 were objects I didn't recognize
                !gameObject.hideFlags.HasFlag(HideFlags.HideInHierarchy);
                    // I don't care about GameObjects *inside* prefabs, just the overall prefab.
        }
    }
}