Add missing override of DeferredConstantExpression in docgen.

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 2a7ead3e932b8f94a08fbecf77f7172d3cf36565..816d4d39fd322db9a3829dbda467043d8089d964 100644
--- a/pkg/docgen/lib/src/models/model_helpers.dart
+++ b/pkg/docgen/lib/src/models/model_helpers.dart
@@ -172,6 +172,12 @@ class AnnotationCreator
                            AnnotationInfo context) {
     return createAnnotation(exp.element, context);
   }
+
+  @override
+  Annotation visitDeferred(DeferredConstantExpression exp,
+                           AnnotationInfo context) {
+    return exp.expression.accept(this, context);
+  }
 }
 
 /// A declaration is private if itself is private, or the owner is private.