Allow application of external mixins to generated dart protos.

lib/src/dart_options.pb.dart

 ///
 //  Generated code. Do not modify.
 ///
 library dart_options_dart_options;
 
 import 'package:protobuf/protobuf.dart';
 
 class DartMixin extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('DartMixin')
     ..a/*<String>*/(1, 'name', PbFieldType.OS)
@@ skipping 60 matching lines @@
 
   List<DartMixin> get mixins => $_get(0, 1, null);
 }
 
 class _ReadonlyImports extends Imports with ReadonlyMessageMixin {}
 
 class Dart_options {
   static final Extension imports = new Extension<Imports>('FileOptions', 'imports', 28125061, PbFieldType.OM, Imports.getDefault, Imports.create);
   static final Extension defaultMixin = new Extension<String>('FileOptions', 'defaultMixin', 96128839, PbFieldType.OS);
   static final Extension mixin = new Extension<String>('MessageOptions', 'mixin', 96128839, PbFieldType.OS);
+  static final Extension overrideGetter = new Extension<bool>('FieldOptions', 'overrideGetter', 28205290, PbFieldType.OB);
+  static final Extension overrideSetter = new Extension<bool>('FieldOptions', 'overrideSetter', 28937366, PbFieldType.OB);
+  static final Extension overrideHasMethod = new Extension<bool>('FieldOptions', 'overrideHasMethod', 28937461, PbFieldType.OB);
+  static final Extension overrideClearMethod = new Extension<bool>('FieldOptions', 'overrideClearMethod', 28907907, PbFieldType.OB);
   static final Extension dartName = new Extension<String>('FieldOptions', 'dartName', 28700919, PbFieldType.OS);
   static void registerAllExtensions(ExtensionRegistry registry) {
     registry.add(imports);
     registry.add(defaultMixin);
     registry.add(mixin);
+    registry.add(overrideGetter);
+    registry.add(overrideSetter);
+    registry.add(overrideHasMethod);
+    registry.add(overrideClearMethod);
     registry.add(dartName);
   }
 }