Ported two more insertion points samples to Dart

@kwalrath @sigmundch @jakemac53 @jmesserly PTAL.
You could use .addAll, .map, and string interpolation here, which might be good to differentiate from js.

nodesAndEntryPoints.addAll(this.children.map((child) => 
    '${child.outerHtml} ------> '
    '${(child.getDestinationInsertionPoints()[0] as Element).outerHtml}'));