Generate the proper error when a constant refers to itself.

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

Index: pkg/analyzer/lib/src/generated/constant.dart
diff --git a/pkg/analyzer/lib/src/generated/constant.dart b/pkg/analyzer/lib/src/generated/constant.dart
index b0b795867375cbc893541bdd656cb104b5ee6187..fccd3603022149198b4aa8e59984f903f06ca8ad 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -1273,12 +1273,15 @@ class ConstantValueComputer {
         referenceGraph.computeTopologicalSort();
     for (List<ConstantEvaluationTarget> constantsInCycle in topologicalSort) {
       if (constantsInCycle.length == 1) {
-        _computeValueFor(constantsInCycle[0]);
-      } else {
-        for (ConstantEvaluationTarget constant in constantsInCycle) {
-          evaluationEngine.generateCycleError(constantsInCycle, constant);
+        ConstantEvaluationTarget constant = constantsInCycle[0];
+        if (!referenceGraph.getTails(constant).contains(constant)) {
+          _computeValueFor(constant);
+          continue;
         }
       }
+      for (ConstantEvaluationTarget constant in constantsInCycle) {
+        evaluationEngine.generateCycleError(constantsInCycle, constant);
+      }
     }
   }