constructor(constants) {
    /**
     * An array of SVG elements that have been created by this object.
     * @type {Array.<!SVGElement>}
     * @private
     */
    this.debugElements_ = [];

    /**
     * The SVG root of the block that is being rendered.  Debug elements will
     * be attached to this root.
     * @type {SVGElement}
     * @private
     */
    this.svgRoot_ = null;

    /**
     * The renderer's constant provider.
     * @type {!Blockly.blockRendering.ConstantProvider}
     * @private
     */
    this.constants_ = constants;
  }
An object that renders rectangles and dots for debugging rendering code.
@param {!Blockly.blockRendering.ConstantProvider} constants The renderer's
    constants.
@package
@constructor