Change temp dir generation on Windows to use UUIDs instead of random numbers

Change temp dir generation on Windows to use UUIDs instead of random numbers

This avoids having to generate random numbers for all platforms as
only Windows was using them. Also when the andom number generator is
deterministic the Windows implementation using random numbers could
end up running out of temporary files.

R=whesse@google.com

BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=2417

[Bill Hesse, 2011-12-13 16:24:27]

LGTM.

Not really happy about the length of the directory name.