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

pkg/analyzer/lib/src/generated/element.dart

Index: pkg/analyzer/lib/src/generated/element.dart
diff --git a/pkg/analyzer/lib/src/generated/element.dart b/pkg/analyzer/lib/src/generated/element.dart
index e0a7e3133f9f71adef55cc3b02c37e870766956a..0339712ff4cf5321a837b2c5fe0e2f73d1e8c9e6 100644
--- a/pkg/analyzer/lib/src/generated/element.dart
+++ b/pkg/analyzer/lib/src/generated/element.dart
@@ -8679,7 +8679,7 @@ class ParameterMember extends VariableMember implements ParameterElement {
  *
  * This class is not intended to be part of the public API for analyzer.
  */
-abstract class PotentiallyConstVariableElement {
+abstract class PotentiallyConstVariableElement implements VariableElementImpl {
   /**
    * If this element represents a constant variable, and it has an initializer,
    * a copy of the initializer for the constant.  Otherwise `null`.