Library Cycle invalidation

Library Cycle invalidation.  Library cycle information on the element model was not being correctly invalidated on incremental updates to source files, since the appropriate dependencies cannot be surfaced in the task model directly.  This CL adds logic to conservatively invalidate the element model information as needed.

BUG=
R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/10915a849b324e3a9f3e2a37fc245709d0eda370

[Leaf, 2015-10-19 22:31:53]

Description was changed from

==========
Library Cycle invalidation

BUG=
==========

to

==========
Library Cycle invalidation.  Library cycle information on the element model was
not being correctly invalidated on incremental updates to source files, since
the appropriate dependencies cannot be surfaced in the task model directly. 
This CL adds logic to conservatively invalidate the element model information as
needed. 

BUG=
==========

[Leaf, 2015-10-19 22:31:54]

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

[Leaf, 2015-10-19 22:32:27]

https://codereview.chromium.org/1413273002/diff/20001/pkg/analyzer/lib/src/ta...
File pkg/analyzer/lib/src/task/driver.dart (right):

https://codereview.chromium.org/1413273002/diff/20001/pkg/analyzer/lib/src/ta...
pkg/analyzer/lib/src/task/driver.dart:481: final List<TargetedResult>
cyclicPath;
Not sure if this is the right way to surface this.  I wrote some debugging code
(see model.dart) that computes an actual concrete path through the
dependencyCycle, which I found made debugging the issue much simpler since there
are often a very large number of edges on each node in the cycle.  I thought it
would be worthwhile to keep that code around.  If preferred though, I can take
it out, or surface it in a different way.

[Brian Wilkerson, 2015-10-19 23:01:40]

LGTM

https://codereview.chromium.org/1413273002/diff/20001/pkg/analyzer/lib/src/ta...
File pkg/analyzer/lib/src/task/driver.dart (right):

https://codereview.chromium.org/1413273002/diff/20001/pkg/analyzer/lib/src/ta...
pkg/analyzer/lib/src/task/driver.dart:481: final List<TargetedResult>
cyclicPath;
If it makes debugging easier without hurting performance in the non-error case,
then it should stay in.

[Leaf, 2015-10-20 00:08:14]

Committed patchset #2 (id:20001) manually as
10915a849b324e3a9f3e2a37fc245709d0eda370 (presubmit successful).