Changed Map to Map.IMap

With the latest haxe/git I was getting the error:

/usr/lib/haxe/lib/mcli/0,1,2-beta/mcli/Dispatch.hx:320: characters 36-71 : Abstract Map has no @:to function that accepts IMap<Dynamic, Dynamic>

Changing the variable declaration to be typed as `Map.IMap` instead of `Map` works for everything except `[]` array access notation, which isn't used here anyway.

This line fixed the problem for me.