Store elements in the constant eval dependency graph rather than AST nodes.

Store elements in the constant eval dependency graph rather than AST nodes.

This paves the way for constant evaluation to be integrated into the
new task model, since the new tasks for constants will use elements as
their targets.

Since the element model makes a distinction between constructor
members and constructor elements, we have to be careful to be
consistent about what kinds of elements are stored in the dependency
graph.  We always store constructor elements in the dependency graph.

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45611

[Paul Berry, 2015-05-07 17:45:32]

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

[Brian Wilkerson, 2015-05-07 18:34:45]

LGTM

[Paul Berry, 2015-05-07 19:38:51]

Committed patchset #1 (id:1) manually as 45611 (presubmit successful).