Use isValidConst flag to distinguish between const / final expressions.

pkg/analyzer/lib/src/summary/link.dart

Index: pkg/analyzer/lib/src/summary/link.dart
diff --git a/pkg/analyzer/lib/src/summary/link.dart b/pkg/analyzer/lib/src/summary/link.dart
index ae63ef8f7a22e93d0adedf785edfb6dc45800d97..0f642efa3e31af68c2bcfd0fd7ac5a37cc1f67d7 100644
--- a/pkg/analyzer/lib/src/summary/link.dart
+++ b/pkg/analyzer/lib/src/summary/link.dart
@@ -2290,8 +2290,8 @@ class TypeInferenceNode extends Node<TypeInferenceNode> {
   void evaluate(bool inCycle) {
     if (inCycle) {
       _inferredType = DynamicTypeImpl.instance;
-    } else if (variableElement.unlinkedVariable.constExpr.isInvalid) {
-      // TODO(paulberry): implement.
+    } else if (!variableElement.unlinkedVariable.constExpr.isValidConst) {
+      // TODO(paulberry): delete this case and fix errors.
       throw new UnimplementedError();
     } else {
       // Perform RPN evaluation of the cycle, using a stack of