the MockFileSystem's implementation of write_binary_file() will "create" the directory for the file automatically; this is bad because it is inconsistent with what the real filesystem does and can mask bugs (e.g., bug 90872). this was originally done for convenience so that you could just say mock_fs.write_text_file('foo/bar/baz.html') when writing your test and not have to worry about creating fake directories. However, we should probably a different way of doing this that doesn't break the semantics for non-test code.