Remove uses of Path class from tests/standalone.

Remove uses of Path class from tests/standalone.

BUG=
R=ajohnsen@google.com

Committed: https://code.google.com/p/dart/source/detail?r=25399

[Bill Hesse, 2013-07-23 14:52:02]

The only remaining tests using Path are path_test.dart, removed by another CL,
and skipping_dart2js_compilations_test.dart, which tests the internal functions
of test.dart, which has not yet been ported from Path to package:path.

[Anders Johnsen, 2013-07-24 07:38:48]

LGTM, if you haven't, try running in checked-mode and the analyzer. It might
help validating all the type changes.

[Bill Hesse, 2013-07-24 07:45:27]

Committed patchset #4 manually as r25399 (presubmit successful).