Remove dead code for tracking pubspec.yaml files.

Remove dead code for tracking pubspec.yaml files.

Now that we are using "pub list-package-dirs" to find the packages
associated with a context, we no longer need to explicitly track the
presence or absence of a "pubspec.yaml" file within the context.

Note that we will have to watch the "pubspec.lock" file in order to
decide when to re-run "pub list-package-dirs", but we'll do this using
a different mechanism, since the decision of which file(s) to watch is
the responsibility of pub, and communicated to us through the JSON
output of "pub list-package-dirs".  This will be addressed in a future
CL.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=37620

[Brian Wilkerson, 2014-06-23 19:45:15]

LGTM

[Paul Berry, 2014-06-23 21:34:30]

Committed patchset #1 manually as r37620 (presubmit successful).