dart:io: Use PathBuffer class for Directory.list on Windows platform.
dart:io: Use PathBuffer class for Directory.list on Windows platform.

BUG=https://code.google.com/p/dart/issues/detail?id=7505

Review URL: https://codereview.chromium.org//12252006

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@18448 260f80e4-7a28-3924-810f-c04153c831b5

computmaxer/pub,cjkao/dart-pub,maxwellpeterson-wf/pub,dart-lang/pub,dart-lang/pub