src/vs/css.build.ts/CSSPluginUtilities/commonFolderPrefix
class CSSPluginUtilities:	public static commonFolderPrefix(fromPath: string, toPath: string): string {
		const prefix = CSSPluginUtilities.commonPrefix(fromPath, toPath);
		const slashIndex = prefix.lastIndexOf('/');
		if (slashIndex === -1) {
			return '';
		}
		return prefix.substring(0, slashIndex + 1);
	}