'EventSystems' does not exist in the namespace 'UnityEngine'...

I'm attempting to use the following command line command to compile an assembly of the code from my project:

 C:/"Program Files"/Unity/Editor/Data/Mono/bin/gmcs -target:library -out:C:/Users/Austin/Desktop/PixelExpanse.dll -recurse:C:/Users/Austin/Desktop/Projects/Repos/trunk/PixelExpanse/SpaceColonyRefactor/Assets/Source/*.cs -d:RUNTIME -r:C:/"Program Files"/Unity/Editor/Data/Managed/UnityEngine.dll

As you can see, I am, I believe, correctly referencing the UnityEngine.dll. The code that would be compiled contains references to UnityEngine.UI and UnityEngine.EventSystems. But when I run the above command, I get the following compile error:

 error CS0234: The type or namespace name 'EventSystems' does not exist in the namespace 'UnityEngine'. Are you missing an assembly reference?

From what I have been able to find through googling, it SEEMS like an error people were getting when using a pre-4.6 assembly, because thats when EventSystems and UI were both introduced. But I don't know how I could be missing that in the dll I'm referencing as Unity 5 is the only version that has ever touched this computer.

Should I be referencing a different assembly?