startChunkedConversion method
override
    Starts a chunked conversion.
The converter works more efficiently if the given sink is a
StringConversionSink.
Returns a chunked-conversion sink that accepts at most one object. It is
an error to invoke add more than once on the returned sink.
Implementation
ChunkedConversionSink<Object?> startChunkedConversion(Sink<String> sink) {
  if (sink is _Utf8EncoderSink) {
    return _JsonUtf8EncoderSink(
      sink._sink,
      _toEncodable,
      JsonUtf8Encoder._utf8Encode(indent),
      JsonUtf8Encoder._defaultBufferSize,
    );
  }
  return _JsonEncoderSink(
    sink is StringConversionSink ? sink : StringConversionSink.from(sink),
    _toEncodable,
    indent,
  );
}