Allow application of external mixins to generated dart protos.

test/protos/dart_options.proto

Index: test/protos/dart_options.proto
diff --git a/test/protos/dart_options.proto b/test/protos/dart_options.proto
index 7a16d818606f5a865c456b856ee29380b84ec3e5..3f75f67134e339f163ba57135ed5c03e677715b8 100644
--- a/test/protos/dart_options.proto
+++ b/test/protos/dart_options.proto
@@ -10,8 +10,31 @@ package dart_options;
 
 import "descriptor_2_5_opensource.proto";
 
+// A mixin to be applied in the 'with' clause of a generated proto in dart.
+message DartMixin {
+  // Class name of the mixin class.
+  optional string name = 1;
+
+  // File from which the mixin class is imported.

[skybrian, 2016/06/23 19:01:42]

"File" => "URI"

+  // The generated Dart code will use this in an import statement.
+  optional string import_from = 2;
+
+  // Used to apply multiple mixins to a proto.
+  // The mixin listed in parent will always be applied before this one,
+  // making the generated proto implement both mixins.
+  optional string parent = 3;
+}
+
 extend google.protobuf.FileOptions {
 
+  // Defines the named mixins to be used on messages in this file.
+  // These mixins are in addition to internally defined mixins (e.g PbMapMixin)
+  // and may override them.
+  // Warning: there is no checking at compile time for name conflicts between
+  // identifiers defined in the mixin and proto-generated identifiers.
+  // Running dartanalyzer on the generated file might find some of them.
+  repeated DartMixin mixins = 28125061;
+
   // Applies the named mixin to all messages in this file.
   // (May be overridden by the "mixin" option on a message.)
   // For now, "PbMapMixin" is the only available mixin.