toCachedSource = source => {
	if (source instanceof CachedSource) {
		return source;
	}
	const entry = cachedSourceMap.get(source);
	if (entry !== undefined) return entry;
	const newSource = new CachedSource(CompatSource.from(source));
	cachedSourceMap.set(source, newSource);
	return newSource;
}
@param {Source} source source
@returns {CachedSource} cached source