message: fix(positioning): don't modify readonly value
  (Positioning.position): element.getBoundingClientRect() returns a read-only value, so don't assign it directly to a variable whose properties are subsequently modified.
diff: @@ -12,7 +12,15 @@ export class Positioning {
  let parentOffset: ClientRect = {width: 0, height: 0, top: 0, bottom: 0, left: 0, right: 0};
  if (this.getStyle(element, 'position') === 'fixed') {
  - elPosition = element.getBoundingClientRect();
  + const bcRect = element.getBoundingClientRect();
  + elPosition = {
  + width: bcRect.width,
  + height: bcRect.height,
  + top: bcRect.top,
  + bottom: bcRect.bottom,
  + left: bcRect.left,
  + right: bcRect.right
  + };
  } else {
  const offsetParentEl = this.offsetParent(element);
  