SourceFile sets up _lineStarts eagerly

[klavs]

The body of the SourceFile.decoded could be executed lazily. It would increase the performance for cases when serialisation of spans is not necessary. For example, doing successful AST transformations without needing to report errors.

source_span/lib/src/file.dart

Lines 73 to 85 in f934308

	SourceFile.decoded(Iterable<int> decodedChars, {url})
	: url = url is String ? Uri.parse(url) : url,
	_decodedChars = new Uint32List.fromList(decodedChars.toList()) {
	for (var i = 0; i < _decodedChars.length; i++) {
	var c = _decodedChars[i];
	if (c == _CR) {
	// Return not followed by newline is treated as a newline
	var j = i + 1;
	if (j >= _decodedChars.length || _decodedChars[j] != _LF) c = _LF;
	}
	if (c == _LF) _lineStarts.add(i + 1);
	}
	}