Fast path Uint8List to _OneByteString conversion #41703
Description
The new UTF-8 decoder, when the input is found to contain only ASCII bytes, simply calls String.fromCharCodes on the desired range of the input Uint8List.
String.fromCharCodes performs a lot of type and range checks (which are redundant in this case) before it gets to the actual copying loop. Using a specialized conversion function without all the checks could significantly speed up the conversion of short strings.