Get the metadata for a given file ID.

@param {string} id - File ID
@returns {Promise} Resolves with an object containing the metadata

@private
function (id) {
    return this._request('GET', BASE_URL + '/drive/v2/files/' + id, {}).then(function (response) {
      if (response.status === 200) {
        return Promise.resolve(JSON.parse(response.responseText));
      } else {
        return Promise.reject("request (getting metadata for " + id + ") failed with status: " + response.status);
      }
    });
  }