Fix/mono environment tests

This tries to fix the problems to run Environment tests under Mono.

    /tmp/boo/tests/BooCompiler.Tests/TypeSystem/Services/DowncastPermissionsTest.cs(61,59): error CS1061: Type `Boo.Lang.Environments.IEnvironment' does not contain a definition for `Run' and no extension method `Run' of type `Boo.Lang.Environments.IEnvironment' could be found (are you missing a using directive or an assembly reference?)