function ResizeObserverEntry(target, rectInit) {
            var contentRect = createReadOnlyRect(rectInit);
            // According to the specification following properties are not writable
            // and are also not enumerable in the native implementation.
            //
            // Property accessors are not being used as they'd require to define a
            // private WeakMap storage which may cause memory leaks in browsers that
            // don't support this type of collections.
            defineConfigurable(this, { target: target, contentRect: contentRect });
        }
Creates an instance of ResizeObserverEntry.

@param {Element} target - Element that is being observed.
@param {DOMRectInit} rectInit - Data of the element's content rectangle.