//遍历所有DLL
foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
{
    //遍历DLL下的所有类
    foreach (var type in assembly.GetTypes())
    {
        if(type.FullName == "UnityEngine.UI.Text")
        {
            return type; //type是Type类型
        }
    }
}

扩展：
DLL可以单独加载

Assembly a  = Assembly.LoadFile("UnityEngine.UI.dll");