Add explicit dependency from analysis_server to mock.

Add explicit dependency from analysis_server to mock.

Previously we were relying on the implicit dependency via the unittest
package, but this dependency was dropped, causing analysis_server to
break.

Eventually we want to stop relying on the mock package, but we're not
ready for that yet.

R=danrubel@google.com

Committed: https://code.google.com/p/dart/source/detail?r=36144

[Paul Berry, 2014-05-14 00:16:38]

Note: this breakage could have been avoided if there had been an analyzer hint
to warn us that we were making use of an implicit package dependency.  I've
filed a ticket for this (dartbug.com/18804).

Note: this breakage would have been detected by the buildbots if they used pub
to do package resolution.  I'm not sure whether it would be beneficial to file a
ticket about that.

[danrubel, 2014-05-14 00:18:45]

lgtm

[Paul Berry, 2014-05-14 00:29:41]

Committed patchset #1 manually as r36144 (presubmit successful).