Allow application of external mixins to generated dart protos.

test/mixin_test.dart

+import 'package:test/test.dart';
+
+import '../out/protos/mixins.pb.dart' as pb;
+import 'package:protoc_plugin/testing/mixins.dart';
+
+void main() {
+  group('Proto with Mixin1', () {
+    pb.Mixin1PB proto;
+
+    setUp(() {
+      proto = new pb.Mixin1PB();
+    });
+
+    test('is a Mixin1', () {
+      expect(proto is Mixin1, isTrue);
+      expect(proto is Mixin2, isFalse);
+    });
+
+    test('implements interface defined by mixins', () {
+      proto.interfaceString = 'test';
+      expect(proto.hasInterfaceString(), isTrue);
+      expect(proto.interfaceString, equals('test'));
+    });
+  });
+
+  group('Proto with Mixin2', () {
+    pb.Mixin2PB proto;
+
+    setUp(() {
+      proto = new pb.Mixin2PB();
+    });
+
+    test('overrides has method', () {
+      expect(proto.hasOverriddenHasMethod(), isFalse);
+      proto.overriddenHasMethod = 'test';
+
+      expect(proto.hasOverriddenHasMethod(), isTrue);
+    });
+  });
+
+  group('Proto without mixins', () {
+    pb.NoMixinPB proto;
+
+    setUp(() {
+      proto = new pb.NoMixinPB();
+    });
+
+    test('is neither Mixin1 nor Mixin2', () {
+      expect(proto is Mixin1, isFalse);
+      expect(proto is Mixin2, isFalse);
+    });
+  });
+
+  group('Proto with Mixin3', () {
+    pb.Mixin3PB proto;
+
+    setUp(() {
+      proto = new pb.Mixin3PB();
+    });
+
+    test('is both Mixin1 (from parent) and Mixin3', () {
+      expect(proto is Mixin1, isTrue);
+      expect(proto is Mixin2, isFalse);
+      expect(proto is Mixin3, isTrue);
+    });
+  });
+}