Modify WorkOrder to use a dependency walking algorithm that handles cycles.

Modify WorkOrder to use a dependency walking algorithm that handles cycles.

There is no functional change in the behavior of the WorkOrder class;
if a cycle is found, we still set the ".exception" field in all the
work items involved in the cycle.

In a follow-up CL I will add the ability to override this behavior
when computing constant values.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45800

[Paul Berry, 2015-05-14 20:37:06]

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

[Brian Wilkerson, 2015-05-14 20:52:04]

LGTM

[Paul Berry, 2015-05-14 22:38:41]

Committed patchset #1 (id:1) manually as 45800 (presubmit successful).