namespace ambiguity in cake

gistfile1.cake

/*
For reference:
my tools in packages.config

<?xml version="1.0" encoding="utf-8"?>
<packages>
    <package id="Cake" version="0.16.2" />
    <package id="WiX" version="3.10.1" />
    <package id="WixSharp.bin" version="1.2.0.0" />
</packages>

*/

#r tools/WixSharp.bin/lib/WixSharp.dll
#r tools/WixSharp.bin/lib/WixSharp.UI.dll
#r tools/WixSharp.bin/lib/WixSharp.Msi.dll
#r tools/WixSharp.bin/lib/BootstrapperCore.dll
#r tools/WixSharp.bin/lib/Microsoft.Deployment.WindowsInstaller.dll

using io = System.IO;
using WixSharp;
using WixSharp.CommonTasks;
using Microsoft.Deployment.WindowsInstaller;

Task("build")
    .Does(() =>
{

    //This call gives me the error even with the alias, using io = System.IO;
    var testfile = new File();

    //This call works as expected
    var anotherfile = new WixSharp.File();

});

RunTarget("build");