function assertArrayContainsString(string, strings) {
  asserts.assertEquals(
      'Expected the string "' + string +
          '" to appear exactly once in the array <' + strings.join(', ') + '>.',
      1, array.count(strings, function(str) {
        return str == string;
      }));
}
Asserts string matches exactly one item in the given array. Useful for
matching elements in an array without guaranteed ordering.

@param {string} string String to match in the array.
@param {!Array<string>} strings Array of strings find match.