code: function firstComposedNode(insertionPoint) {
      var n$ = insertionPoint.assignedNodes({flatten: true});
      var root = this.getRootNode(insertionPoint);
      for (var i=0, l=n$.length, n; (i<l) && (n=n$[i]); i++) {
        // means that we're composed to this spot.
        if (root.isFinalDestination(insertionPoint, n)) {
          return n;
        }
      }
    }
docs: TODO(sorvell): This will fail if distribution that affects this question is pending; this is expected to be exceedingly rare, but if the issue comes up, we can force a flush in this case.