@Override
  public boolean isAbsolute() {
    return root != null;
  }
Equivalent to {@link #getFileSystem()} but with a return type of {@code JimfsFileSystem}.
{@code getFileSystem()}'s return type is left as {@code FileSystem} to make testing paths
easier (as long as methods that access the file system in some way are not called, the file
system can be a fake file system instance).
jimfs/src/main/java/com/google/common/jimfs/JimfsPath.java