src/vs/workbench/api/common/extHostTypes.ts/SnippetString/isSnippetString
class SnippetString:	static isSnippetString(thing: any): thing is SnippetString {
		if (thing instanceof SnippetString) {
			return true;
		}
		if (!thing) {
			return false;
		}
		return typeof (<SnippetString>thing).value === 'string';
	}