Avoid canonicalization issues in the pub web socket API tests.
Avoid canonicalization issues in the pub web socket API tests.

We were using [path.absolute], which can return a different string
than pub's understanding of the current working directory on OS X bots
where /tmp is symlinked to /private/tmp.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//200723009

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@34094 260f80e4-7a28-3924-810f-c04153c831b5

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