Fix path to only use public interface in the implementation of methods.
Fix path to only use public interface in the implementation of methods.

This allows other people to implement the Path interface and have the
dart:io implementation interact with those Path objects.

R=sgjesse@google.com
BUG=dartbug.com/5913

Review URL: https://codereview.chromium.org//11189125

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@13923 260f80e4-7a28-3924-810f-c04153c831b5

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