I eventually fixed it by copying the `Microsoft.SharePoint.Client.ServerRuntime` dll from an on premises SharePoint farm and adding a reference to it. Then it requires `Microsoft.Office.Project.Schema` which is missing from the current version of the SDK. I found this dll in an old RTM version of the SDK here: http://blogs.msdn.com/cfs-filesystemfile.ashx/__key/communityserver-components-postattachments/00-10-40-35-79/REDIST.zip