This is probably a frivolous issue, but looking at HEAD there are ~3000 unused using statements spread across >800 files spanning the project. It would be nice to clean these up a bit.

Not picking on any particular class, but lets take /src/XMakeBuildEngine/BackEnd/BuildManager/BuildRequestData.cs

This:

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

using Microsoft.Build.Collections;
using Microsoft.Build.Evaluation;
using Microsoft.Build.Shared;

can be shortened to this:

using System;
using System.Collections.Generic;

using Microsoft.Build.Collections;
using Microsoft.Build.Shared;