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

originating from com.sun.tools.javac.processing.JavacFiler.warnIfUnclosedFiles.

It works by opening and closing an OutputStream on the dummy JavaFileObject,
thereby clearing the dummy filename from the JavacFilers internal openTypeNames set.

Note, this will result in a an empty dummy file, e.g.  build/generated/source/apt/debug/dummy1407426008266.java