Fix pub's listDir on Windows.
Fix pub's listDir on Windows.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//348463003

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@37471 260f80e4-7a28-3924-810f-c04153c831b5

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