Allow application of external mixins to generated dart protos.

test/protos/mixins.proto

+syntax = "proto2";
+
+import "dart_options.proto";
+
+option (dart_options.imported_mixins) = {
+  mixins: [{

[skybrian, 2016/06/28 00:13:08]

The bad news is that this syntax for initializing a repeated field doesn't work
with protoc 2.5.0 either.

mixins.proto:5:41: Error while parsing option value for "imported_mixins":
Expected "{", found "[".

However, I can still populate this mixin list with a single mixin if I comment
out the other two, so I guess it's not a total loss.

I think we should require a newer release of protoc for this feature.

+    name: "Mixin1"
+    import_from: "package:protoc_plugin/testing/mixins.dart"
+  },
+  {
+   name: "Mixin2"
+   import_from: "package:protoc_plugin/testing/mixins.dart"
+  },
+  {
+    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;
+}