Fix bug in sign-extending varints
Fix bug in sign-extending varints

TBR=sigmund@google.com
BUG=

Review URL: https://chromiumcodereview.appspot.com//26227003

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