Fix a corner case of constant constructor dependency computation.

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 499ce5081f0fdeb33605d5dba3b169530a8452d8..ddf4a3a3b0243c7ece15a3bb10d07d4816b1f07c 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -457,7 +457,7 @@ class ConstantEvaluationEngine {
         if (superclass != null && !superclass.isObject) {
           ConstructorElement unnamedConstructor = ConstantEvaluationEngine
               ._getConstructorBase(superclass.element.unnamedConstructor);
-          if (unnamedConstructor != null) {
+          if (unnamedConstructor != null && unnamedConstructor.isConst) {
             callback(unnamedConstructor);
           }
         }