Folder for non-editor dll

I need System.Xml.Linq.dll in my web build, but if I just copy this DLL into /Assets/Plugins folder there're a conflict between Assembly-CSharp-Editor and Assembly-CSharp-firstpass. It happens becous first assembly using .dll from "Unity\\Editor\\Data\\Mono\\lib\\mono\\unity" and it's also loaded from the Plugins folder so it's "defined multiple times". But if I remove this dll from Plugins then second assembly can't find XmlLinq becouse it's removed from default dll set in "Unity\\Editor\\Data\\Mono\\lib\\mono\\unity_web". I could solve it by puttin System.Xml.Linq.dll inside unity_web, but I don't want use this dirty hack. Does Unity have some special folder for this case?