I believe the class ConfigTree needs the TreeAttribute added.

Without it the packages causes System.InvalidOperationException: Sequence contains no elements while installing this package as well as some others.  I was unable to install courier correctly because this error stops the install somewhere along the lines.

Here is the code in the umbraco core that fails:

umbraco.BusinessLogic.ApplicationTreeRegistrar

public ApplicationTreeRegistrar()

        { ....

var items = types.Select(x => new Tuple<Type, TreeAttribute>(x, x.GetCustomAttributes<TreeAttribute>(false).Single())).Where(x => ApplicationTree.getByAlias(x.Item2.Alias) == null);

.. }

The .Single fails because it is missing.