// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp,
(See below)

  interface DateTimeFormatOptions {
        localeMatcher?: 'lookup' | 'best fit';
        weekday?: 'long' | 'short' | 'narrow';
        era?:  'long' | 'short' | 'narrow';
        year?: 'numeric' | '2-digit';
        month?: 'numeric' | '2-digit' | 'long' | 'short' | 'narrow';
        day?: 'numeric' | '2-digit';
        hour?: 'numeric' | '2-digit';
        minute?: 'numeric' | '2-digit';
        second?: 'numeric' | '2-digit';
        timeZoneName?: 'long' | 'short';
        formatMatcher?: 'basic' | 'best fit';
        hour12?: boolean;
        timeZone?: string; // this is more complicated than the others, not sure what I expect here
    }

 interface DateTimeFormatOptions {
        localeMatcher?: string;
        weekday?: string;
        era?: string;
        year?: string;
        month?: string;
        day?: string;
        hour?: string;
        minute?: string;
        second?: string;
        timeZoneName?: string;
        formatMatcher?: string;
        hour12?: boolean;
        timeZone?: string;
    }