[Mono-list] Why are extension method dependent on .NET 3.5?

[Jiří Zárevúcký]

Hello, I don't know whether there is some technological concept behind
this, but I always found that fact very annoying.
As far as I know, extension methods are purely a syntactic sugar,
aren't they? As such, there should be no runtime dependence on the
.NET 3.5, because everything I write using an extension method can be
very simply written as a normal static call. Neither should it have
(e.g. there is no reason for) compile-time dependence on
System.Core.dll. Can someone explain to me, then, why is it dependent?