How to reproduce:

• create a directory
• create a symlink to this directory
• want to create a file via Filesystem::dumpFile('symlink/filename', $someContent)

Expected: File is created.
Result: IOException mkdir(): File exists.

Reason:

    public function dumpFile($filename, $content)
    {
        $dir = dirname($filename);

        if (!is_dir($dir)) {
            $this->mkdir($dir);
        }

is_dir() does not account for symlinks.