an array of property names. Accepts either arrays
     * of path parts or strings.
     *
     * Example:
     *
     * ```
     * Polymer.Path.split(['foo.bar', 0, 'baz'])  // ['foo', 'bar', '0', 'baz']
     * Polymer.Path.split('foo.bar.0.baz')        // ['foo', 'bar', '0', 'baz']
     * ```
     *
     * @memberof Polymer.Path
     * @param {string | !Array<string|number>} path Input path
     * @return {!Array<string>} Array of path parts
     * @this {Path}
     * @suppress {checkTypes}
     */
    spl