Allow application of external mixins to generated dart protos.

test/protos/mixins.proto

Index: test/protos/mixins.proto
diff --git a/test/protos/mixins.proto b/test/protos/mixins.proto
new file mode 100644
index 0000000000000000000000000000000000000000..eca47a1817a72fe3d3063458162b62fd9a2acc1c
--- /dev/null
+++ b/test/protos/mixins.proto
@@ -0,0 +1,39 @@
+syntax = "proto2";
+
+import "dart_options.proto";
+
+option (dart_options.mixins) = {
+  name: "Mixin1"
+  import_from: "package:protoc_plugin/testing/mixins.dart"
+};
+option (dart_options.mixins) = {
+  name: "Mixin2"
+  import_from: "package:protoc_plugin/testing/mixins.dart"
+};
+option (dart_options.mixins) = {
+  name: "Mixin3"
+  import_from: "package:protoc_plugin/testing/mixins.dart"
+  parent: "Mixin1"
+};
+
+option (dart_options.default_mixin) = "Mixin1";
+
+message NoMixinPB {
+  option (dart_options.mixin) = "";
+}
+
+message Mixin1PB {
+  optional string interface_string = 1;
+}
+
+message Mixin2PB {
+  option (dart_options.mixin) = "Mixin2";
+
+  optional string overridden_has_method = 3;
+}
+
+message Mixin3PB {
+  option (dart_options.mixin) = "Mixin3";
+
+  optional string interface_string = 1;
+}