Rework handling of potentially-constant variables.

Rework handling of potentially-constant variables.

Previously, we had a mixin class PotentiallyConstVariableElement to
represent variables which might be constant.  This left some holes in
the type hierarchy which didn't implement
PotentiallyConstVariableElement but nonetheless could be reached by
constant evaluation code in the event of errors in the code being
analyzed, resulting in an invalid cast exception.

This CL drops PotentiallyConstVariableElement and moves its one getter
to VariableElementImpl so that it will be available to all variable
elements.  This allows us to avoid a lot of casts, including the one
that previously caused the exception.

Fixes #24645.

R=scheglov@google.com

Committed: https://github.com/dart-lang/sdk/commit/1a7528174263af0a95d7edf51c01a2b3d1b568ff

[Paul Berry, 2015-10-20 18:03:13]

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

[scheglov, 2015-10-20 18:06:33]

LGTM

[Paul Berry, 2015-10-20 19:37:14]

Committed patchset #1 (id:1) manually as
1a7528174263af0a95d7edf51c01a2b3d1b568ff (presubmit successful).