This change causes RawWrite to buffer upto 128 bytes or until a line is reached. This helps group calls into more readable lines.
Ideally you'd buffer multiple short lines when they're all in the same call and do a single __sanitizer_log_write call.
if lastNewline was null now it's line and so this writes a count of 0.
This seems like an odd way to describe it. The behavior is just "Flush all complete lines before returning".