Workaround #6986.
Workaround #6986.

This avoids trying to read a non-existent lockfile. On some
non-English locales, this fails when trying to parse the error
message coming from Windows.
BUG=6986

Review URL: https://codereview.chromium.org//11477012

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@15992 260f80e4-7a28-3924-810f-c04153c831b5

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