src/vs/editor/contrib/snippet/snippetParser.ts/Variable/toTextmateString
class Variable:	toTextmateString(): string {
		let transformString = '';
		if (this.transform) {
			transformString = this.transform.toTextmateString();
		}
		if (this.children.length === 0) {
			return `\${${this.name}${transformString}}`;
		} else {
			return `\${${this.name}:${this.children.map(child => child.toTextmateString()).join('')}${transformString}}`;
		}
	}