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