Change how to set the Dart name of a field

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";
 
 extend google.protobuf.FileOptions {
 
   // 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;
+}