asUtf8Sink method
override
    Returns this as a sink that accepts UTF-8 input.
If used, this method must be the first and only call to this. It
invalidates this. All further operations must be performed on the result.
Implementation
ByteConversionSink asUtf8Sink(bool allowMalformed) {
  return new _Utf8ConversionSink(this, allowMalformed);
}