Change dart:io Platform.script to return a Uri.

Change dart:io Platform.script to return a Uri.

BUG=
R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=29651

[Bill Hesse, 2013-10-31 02:27:30]

This change was committed as r29619 and r29624, and the buildbot was green.  It
is reverted pending review, for final commit after an LGTM.

The safe change to any use of Platform.script is to update it
to Platform.script.toFilePath(), but in many cases we could do much better.

[Søren Gjesse, 2013-10-31 07:08:53]

LGTM

[Søren Gjesse, 2013-10-31 07:29:57]

Committed patchset #2 manually as r29651 (presubmit successful).