WritableStream API exposed for controllers.
function WritableStreamAddWriteRequest(stream) {
  assert(IsWritableStreamLocked(stream) === true);
  assert(stream._state === 'writable');

  const promise = new Promise((resolve, reject) => {
    const writeRequest = {
      _resolve: resolve,
      _reject: reject
    };

    stream._writeRequests.push(writeRequest);
  });

  return promise;
}