src/vs/editor/contrib/snippet/snippetParser.ts/Transform/_replace
class Transform:	private _replace(groups: string[]): string {
		let ret = '';
		for (const marker of this._children) {
			if (marker instanceof FormatString) {
				let value = groups[marker.index] || '';
				value = marker.resolve(value);
				ret += value;
			} else {
				ret += marker.toString();
			}
		}
		return ret;
	}