Don't do folder existence checks when parsing pub list results.

Don't do folder existence checks when parsing pub list results.

It's just a waste of time since we already to folder existence checks
when resolving package URI's.  Besides, all the folders output by pub
list should always exist except in pathological situations.

Improves total analysis time by about 2%.

R=danrubel@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44137

[Paul Berry, 2015-02-25 22:00:54]

paulberry@google.com changed reviewers:
+ danrubel@google.com

[danrubel, 2015-02-25 23:52:26]

lgtm

[Paul Berry, 2015-03-02 18:29:07]

Committed patchset #1 (id:1) manually as 44137 (presubmit successful).