Thinking why the hell would you want to do this? , I was just surprised and found what really could be a solution!
Cocotron
Cocotron is an open source project whose goal is to implement the cross-platform Objective-C API, similar to that described in the documentation of Apple Inc. Cocoa This includes AppKit, Foundation, the Objective-C runtime, and supporting APIs such as CoreGraphics and CoreFoundation.
Also see this blog post: Win-win with Cocotron and Xcode 4.3 - Code for Mac, Build for Windows (Part 1)
Although the last entry on Cocotron's website was from 2010, so it may or may not be live
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