function assocIndexOf(array, key) {
  var length = array.length;
  while (length--) {
    if (eq(array[length][0], key)) {
      return length;
    }
  }
  return -1;
}
Gets the index at which the `key` is found in `array` of key-value pairs.

@private
@param {Array} array The array to inspect.
@param {*} key The key to search for.
@returns {number} Returns the index of the matched value, else `-1`.