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;
+}
+
 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.

[skybrian, 2016/06/22 20:29:53]

Need a warning here: there is no error-checking for inadvertent name conflicts
between mixin fields and protobuf fields. (But dartanalyzer might find some of
them.)

I think we need the warning because you are writing a tool for this but it won't
be open source.

[frederikmutzel, 2016/06/23 12:30:16]

Done.

+  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.
   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 {
 
+  // Specifies a field to override getters/setters from the inherited mixin.

[skybrian, 2016/06/22 20:29:53]

I don't see any implementation of "override" - that is, in this patch it doesn't
do anything.

One thing it could do is add the @override annotation to the getter, setter, and
clear method, allowing some errors to be detected with dartanalyzer. But we
probably need separate options to add @override to the getter, setter, and clear
methods.

Maybe leave it out of this patch?

[frederikmutzel, 2016/06/23 12:30:16]

Right, removed for now.

+  optional bool override = 28205290;
+
   // 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;
 }