Currently, TypeMixer uses a fixed type discovery serve that loads all assemblies, including system assemblies. This causes problems when a system assembly has an unresolved dependency (e.g., Windows 8/.NET 3.5 - see http://social.technet.microsoft.com/Forums/en-US/w8itproappcompat/thread/d570f7cb-1ee2-440b-80ca-42a8cde4e085). In addition, it disables customization of Type Discovery.
Therefore, TypeMixer could be changed to use the ContextAwareTypeDiscoveryUtility that is also used by the rest of the application.
TBD: It's actually on purpose that TypeMixer uses a LoadAllAssemblyLoaderFilter - that way, concrete types can be pregenerated for system assemblies (e.g., for Pages and Controls - required for pre-mixed ASP.NET). If we change this to the ContextAwareTypeDiscoveryUtility, those types won't be premixed.