Use UnaryOperator and BinaryOperator in ConstantSystem.

pkg/docgen/lib/src/models/model_helpers.dart

Index: pkg/docgen/lib/src/models/model_helpers.dart
diff --git a/pkg/docgen/lib/src/models/model_helpers.dart b/pkg/docgen/lib/src/models/model_helpers.dart
index d5876815d4487eb6a6fce478e7c509ab0857e072..2a7ead3e932b8f94a08fbecf77f7172d3cf36565 100644
--- a/pkg/docgen/lib/src/models/model_helpers.dart
+++ b/pkg/docgen/lib/src/models/model_helpers.dart
@@ -101,6 +101,12 @@ class AnnotationCreator
   }
 
   @override
+  Annotation visitIdentical(IdenticalConstantExpression exp,
+                            AnnotationInfo context) {
+    return null;
+  }
+
+  @override
   Annotation visitConcatenate(ConcatenateConstantExpression exp,
                               AnnotationInfo context) {
     return null;