File's source does not take some cases of absolute paths as absolute paths.

Using an absolute path that starts with either forward or backward slash is not accepted.
It happens because Java's File.isAbsolute() does not recognize paths starting at either forward or backward as absolute paths on Windows. However, those prefixes are acceptable on Windows.
This function is used at main/java/org/apache/maven/plugin/assembly/archive/phase/FileItemAssemblyPhase.java, line 76 to know if the source is either absolute or relative.