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 DartOptions extends GeneratedMessage {
+  static final BuilderInfo _i = new BuilderInfo('DartOptions')
+    ..hasRequiredFields = false
+  ;
+
+  DartOptions() : super();
+  DartOptions.fromBuffer(List<int> i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromBuffer(i, r);
+  DartOptions.fromJson(String i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromJson(i, r);
+  DartOptions clone() => new DartOptions()..mergeFromMessage(this);
+  BuilderInfo get info_ => _i;
+  static DartOptions create() => new DartOptions();
+  static PbList<DartOptions> createRepeated() => new PbList<DartOptions>();
+  static DartOptions getDefault() {
+    if (_defaultInstance == null) _defaultInstance = new _ReadonlyDartOptions();
+    return _defaultInstance;
+  }
+  static DartOptions _defaultInstance;
+  static void $checkItem(DartOptions v) {
+    if (v is !DartOptions) checkItemFailed(v, 'DartOptions');
+  }
+}
+
+class _ReadonlyDartOptions extends DartOptions with ReadonlyMessageMixin {}
+
+class DartMixin extends GeneratedMessage {
+  static final BuilderInfo _i = new BuilderInfo('DartMixin')
+    ..a/*<String>*/(1, 'name', PbFieldType.OS)
+    ..a/*<String>*/(2, 'importFrom', PbFieldType.OS)
+    ..a/*<String>*/(3, 'parent', PbFieldType.OS)
+    ..hasRequiredFields = false
+  ;
+
+  DartMixin() : super();
+  DartMixin.fromBuffer(List<int> i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromBuffer(i, r);
+  DartMixin.fromJson(String i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromJson(i, r);
+  DartMixin clone() => new DartMixin()..mergeFromMessage(this);
+  BuilderInfo get info_ => _i;
+  static DartMixin create() => new DartMixin();
+  static PbList<DartMixin> createRepeated() => new PbList<DartMixin>();
+  static DartMixin getDefault() {
+    if (_defaultInstance == null) _defaultInstance = new _ReadonlyDartMixin();
+    return _defaultInstance;
+  }
+  static DartMixin _defaultInstance;
+  static void $checkItem(DartMixin v) {
+    if (v is !DartMixin) checkItemFailed(v, 'DartMixin');
+  }
+
+  String get name => $_get(0, 1, '');
+  void set name(String v) { $_setString(0, 1, v); }
+  bool hasName() => $_has(0, 1);
+  void clearName() => clearField(1);
+
+  String get importFrom => $_get(1, 2, '');
+  void set importFrom(String v) { $_setString(1, 2, v); }
+  bool hasImportFrom() => $_has(1, 2);
+  void clearImportFrom() => clearField(2);
+
+  String get parent => $_get(2, 3, '');
+  void set parent(String v) { $_setString(2, 3, v); }
+  bool hasParent() => $_has(2, 3);
+  void clearParent() => clearField(3);
+}
+
+class _ReadonlyDartMixin extends DartMixin with ReadonlyMessageMixin {}
+
 class Dart_options {
+  static final Extension mixins = new Extension<DartMixin>.repeated('FileOptions', 'mixins', 28125061, PbFieldType.PM, DartMixin.$checkItem, DartMixin.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 override = new Extension<bool>('FieldOptions', 'override', 28205290, PbFieldType.OB);
   static final Extension dartName = new Extension<String>('FieldOptions', 'dartName', 28700919, PbFieldType.OS);
   static void registerAllExtensions(ExtensionRegistry registry) {
+    registry.add(mixins);
     registry.add(defaultMixin);
     registry.add(mixin);
+    registry.add(override);
     registry.add(dartName);
   }
 }