printString = function(str) {
        'use strict';
        var startIndex = Math.max(0, commonPrefix - 2);
        var endIndex = Math.min(str.length, str.length - (commonSuffix - 2));
        return (startIndex > 0 ? '...' : '') +
            str.substring(startIndex, endIndex) +
            (endIndex < str.length ? '...' : '');
      }
@param {*} expected The expected value.
@param {*} actual The actual value.
@return {string} A failure message of the values don't match.
@private