Change how to set the Dart name of a field

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 aa9942ef069bde67407eac0a0727f78c94f2c62b..7a16d818606f5a865c456b856ee29380b84ec3e5 100644
--- a/test/protos/dart_options.proto
+++ b/test/protos/dart_options.proto
@@ -25,3 +25,10 @@ extend google.protobuf.MessageOptions {
   // 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;
+}