src/vs/base/common/htmlContent.ts/isMarkdownString
export function isMarkdownString(thing: any): thing is IMarkdownString {
	if (thing instanceof MarkdownString) {
		return true;
	} else if (thing && typeof thing === 'object') {
		return typeof (<IMarkdownString>thing).value === 'string'
			&& (typeof (<IMarkdownString>thing).isTrusted === 'boolean' || (<IMarkdownString>thing).isTrusted === undefined)
			&& (typeof (<IMarkdownString>thing).supportThemeIcons === 'boolean' || (<IMarkdownString>thing).supportThemeIcons === undefined);
	}
	return false;
}