Allow application of external mixins to generated dart protos.

test/protos/dart_options.proto

 // Experimental options controlling Dart code generation.
 syntax = "proto2";
 
 package dart_options;
 
 // This file must be modified for google internal use,
 // because custom options only work when the package name
 // agrees with the version of protoc we are using.
 // (The import statement and "google.protobuf." prefix need to be changed.)
 
 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.
+  // 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;
+}
+
+// Defines mixins imported from dart files to be used on messages in this file.
+message ImportedMixins {

[skybrian, 2016/06/27 23:33:37]

On second thought, maybe just "Imports". This will allow us to add non-mixin
imports to the same message if we have some reason to do that.

+
+  // The DartMixins to be imported.
+  repeated DartMixin mixins = 1;
+}
+
 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.
+  optional ImportedMixins imported_mixins = 28125061;

[skybrian, 2016/06/27 23:33:37]

rename to "imports". Most of the comment can be moved to the "mixins" field.

+
   // 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.
   optional string default_mixin = 96128839;
 }
 
 extend google.protobuf.MessageOptions {
 
   // Applies the named mixin.
   // For now, "PbMapMixin" is the only available mixin.
   // The empty string can be used to turn off mixins for this message.
   optional string mixin = 96128839;
 }
 
 extend google.protobuf.FieldOptions {
 
   // Uses the given name for getters, setters and as suffixes for has/clear
   // methods in the generated Dart file. Should be lowerCamelCase.
   optional string dart_name = 28700919;
 }