Fix File::OpenUri on Windows.
Fix File::OpenUri on Windows.

This is follow-up to c5c18a0ace7e4cb6f827ad3a5b80fae15d3e7b3f.

Bug: dartbug.com/33793
Change-Id: Ic281c11ea5a2c4e10cdccb0617a12b6dfbda1c68
Reviewed-on: https://dart-review.googlesource.com/64243
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk