improve annotation argument completion - from https://github.com/dart-lang/sdk/pull/25099

pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart

Index: pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart b/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
index 4f7780d5f2626643deeb2d90b3587591cbc284c4..69d00446ad64a5f845b87af36b10edb3c116fac2 100644
--- a/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/arglist_contributor.dart
@@ -198,6 +198,12 @@ class _ArgSuggestionBuilder {
         }
       }
     }
+    if (parent is Annotation) {
+      Element element = parent.element;
+      if (element is ExecutableElement) {
+        _addSuggestions(element.parameters);
+      }
+    }
     return new Future.value(false);
   }