Ensure that renames fail atomically in pub.
Ensure that renames fail atomically in pub.

R=rnystrom@google.com
BUG=12436

Review URL: https://codereview.chromium.org//22799008

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@26150 260f80e4-7a28-3924-810f-c04153c831b5

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