Minor refactoring in `_FieldSet` (#728)
Minor refactoring in `_FieldSet` (#728)

- Make `isExtension` argument of `_mergeField` `required`: the argument
  is always passed so no need to handle null case in the method.

- Remove mutable `mustClone` variable in `_mergeField`

- Document exception behavior of `_validateField`
google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/dart-protobuf,google/protobuf.dart,google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/protobuf,dart-lang/protobuf