 the <a href="../../dart-convert/AsciiCodec/decode.html">decode</a> method and the converter
returned by <a href="../../dart-convert/AsciiCodec/decoder.html">decoder</a> will default to allowing invalid values.
If allowing invalid values, the values will be decoded into the Unicode
Replacement character (U+FFFD). If not, an exception will be thrown.
Calls to the <a href="../../dart-convert/AsciiCodec/decode.html">decode</a> method can choose to override this default.</p>
<p>Encoders will not accept invalid 