Allow application of external mixins to generated dart protos.

test/protos/dart_options.proto

 // Experimental options controlling Dart code generation.
 syntax = "proto2";
 
 package dart_options;
 
 // This file must be modified for Google internal use,
 // because custom options only work when the package name
 // agrees with the version of protoc we are using.
 // (The import statement and "google.protobuf." prefix need to be changed.)
 
@@ skipping 41 matching lines @@
 
 extend google.protobuf.MessageOptions {
 
   // Applies the named mixin.
   // For now, "PbMapMixin" is the only available mixin.
   // The empty string can be used to turn off mixins for this message.
   optional string mixin = 96128839;
 }
 
 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.
   optional string dart_name = 28700919;
 }