SpanScanner constructor
Creates a new SpanScanner that starts scanning from position.
sourceUrl is used as SourceLocation.sourceUrl for the returned
FileSpans as well as for error reporting. It can be a String, a
Uri, or null.
Implementation
SpanScanner(super.string, {super.sourceUrl, super.position})
: _sourceFile = SourceFile.fromString(string, url: sourceUrl);