Fix NPE when computing dependencies of enum constants

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 fa5360b8d87769eb4ed2ff81607308a243b8ea56..499ce5081f0fdeb33605d5dba3b169530a8452d8 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -418,7 +418,10 @@ class ConstantEvaluationEngine {
         }
       }
     } else if (constant is PotentiallyConstVariableElement) {
-      constant.constantInitializer.accept(referenceFinder);
+      Expression initializer = constant.constantInitializer;
+      if (initializer != null) {
+        initializer.accept(referenceFinder);
+      }
     } else if (constant is ConstructorElementImpl) {
       constant.isCycleFree = false;
       ConstructorElement redirectedConstructor =