Convert analysis starting point to an absolute path before analyzing.

Convert analysis starting point to an absolute path before analyzing.

This ensures that if the analysis starting point is part of an import
loop, then once the analysis engine reaches the starting point again,
it will recognize that it's the same file.

BUG=dartbug.com/21440
R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=41473

[Paul Berry, 2014-11-03 21:45:46]

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

[Brian Wilkerson, 2014-11-03 21:58:33]

LGTM

[Paul Berry, 2014-11-03 22:42:57]

Committed patchset #1 (id:1) manually as 41473 (presubmit successful).