function getModernOffsetsFromPoints(outerNode, anchorNode, anchorOffset, focusNode, focusOffset) {
    var length = 0;
    var start = -1;
    var end = -1;
    var indexWithinAnchor = 0;
    var indexWithinFocus = 0;
    var node = outerNode;
    var parentNode = null;

    outer: while (true) {
      var next = null;

      while (true) {
        if (node === anchorNode && (anchorOffset === 0 || node.nodeType === TEXT_NODE)) {
          start = length + anchorOffset;
        }

        if (node === focusNode && (focusOffset === 0 || node.nodeType === TEXT_NODE)) {
          end = length + focusOffset;
        }

        if (node.nodeType === TEXT_NODE) {
          length += node.nodeValue.length;
        }

        if ((next = node.firstChild) === null) {
          break;
        } // Moving from `node` to its first child `next`.


        parentNode = node;
        node = next;
      }

      while (true) {
        if (node === outerNode) {
          // If `outerNode` has children, this is always the second time visiting
          // it. If it has no children, this is still the first loop, and the only
          // valid selection is anchorNode and focusNode both equal to this node
          // and both offsets 0, in which case we will have handled above.
          break outer;
        }

        if (parentNode === anchorNode && ++indexWithinAnchor === anchorOffset) {
          start = length;
        }

        if (parentNode === focusNode && ++indexWithinFocus === focusOffset) {
          end = length;
        }

        if ((next = node.nextSibling) !== null) {
          break;
        }

        node = parentNode;
        parentNode = node.parentNode;
      } // Moving from `node` to its next sibling `next`.


      node = next;
    }

    if (start === -1 || end === -1) {
      // This should never happen. (Would happen if the anchor/focus nodes aren't
      // actually inside the passed-in node.)
      return null;
    }

    return {
      start: start,
      end: end
    };
  }
Returns {start, end} where `start` is the character/codepoint index of
(anchorNode, anchorOffset) within the textContent of `outerNode`, and
`end` is the index of (focusNode, focusOffset).

Returns null if you pass in garbage input but we should probably just crash.

Exported only for testing.