Allow application of external mixins to generated dart protos.

test/protos/dart_options.proto

Index: test/protos/dart_options.proto
diff --git a/test/protos/dart_options.proto b/test/protos/dart_options.proto
index 80565186350af9a55e6f606d170d3df01808aa59..265dd39338d8760b72f065385ef13b62f1f9ef1c 100644
--- a/test/protos/dart_options.proto
+++ b/test/protos/dart_options.proto
@@ -59,6 +59,19 @@ extend google.protobuf.MessageOptions {
 }
 
 extend google.protobuf.FieldOptions {
+  // Adds @override annotation to the field's getter (for use with mixins).
+  optional bool override_getter = 28205290;
+
+  // Adds @override annotation to the field's setter (for use with mixins).
+  optional bool override_setter = 28937366;
+
+  // Adds @override annotation to the field's hasX() method (for use with
+  // mixins).
+  optional bool override_has_method = 28937461;
+
+  // Adds @override annotation to the field's clearX() method (for use with
+  // mixins).
+  optional bool override_clear_method = 28907907;
 
   // Uses the given name for getters, setters and as suffixes for has/clear
   // methods in the generated Dart file. Should be lowerCamelCase.