Fix an incorrect return type in ReadonlyMessageMixin
Fix an incorrect return type in ReadonlyMessageMixin

This is harmless because createRepeatedField should never be
called for a read-only message, but dart2dart warns about it.

BUG=
R=sgjesse@google.com

Review URL: https://chromiumcodereview.appspot.com//1364943002.

google/protobuf.dart,dart-lang/protobuf,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/dart-protobuf,collinjackson/dart-protobuf,dart-lang/protobuf,google/protobuf.dart,dart-lang/dart-protobuf,google/protobuf.dart