startChunkedConversion method
override
    Starts a chunked conversion.
The returned sink serves as input for the long-running conversion. The
given sink serves as output.
Implementation
StringConversionSink startChunkedConversion(Sink<String> sink) {
  if (sink is! StringConversionSink) {
    sink = new StringConversionSink.from(sink);
  }
  return new _HtmlEscapeSink(this, sink);
}