Support Windows share paths in the Path class.

Support Windows share paths in the Path class.

Use that to implement Directory.create(recursive: true) in a way that
works with shares.

The Path implementation has an additional piece of state. The
actual path is treated like an absolute path except for
toNativePath which adds back the extra backslash. The additional
state is preserved in operations that produce new paths.

R=sgjesse@google.com
BUG=dartbug.com/6904

Committed: https://code.google.com/p/dart/source/detail?r=15385

[Søren Gjesse, 2012-11-27 11:50:31]

lgtm