Headers type not available

[ry]

Adding the following to js/headers_test.ts break it

test(function headerTypesAvailable() {
  function newHeaders(): Headers {
    return new Headers();
  }
  const headers = newHeaders();
  assert(headers instanceof Headers);
});

Results in the following error:

> ./out/debug/deno  js/headers_test.ts
Compiling /Users/rld/src/deno/js/headers_test.ts
/Users/rld/src/deno/js/headers_test.ts:176:26 - error TS2304: Cannot find name 'Headers'.

176   function newHeaders(): Headers {
                             ~~~~~~~