Silence the JavacFiler warning 'compiler.warn.proc.unclosed.type.files'

This prevents warnings like:

   warning: Unclosed files for the types '[dummy1407426008266]'; these types will not undergo annotation processing

Files created via the createResource method are not registered for annotation
processing (as opposed to files created by the createSourceFile method), so we
avoid the JavacFiler warning and we don't have to open/close a stream.