function testMap1() {
  var nodes = new Array(6);
  for (var i = 0; i < nodes.length; i++) {
    nodes[i] = new TreeNode_(i);
  }

  nodes[2].addChild(nodes[3]);
  nodes[1].addChild(nodes[2]);
  nodes[1].addChild(nodes[4]);
  nodes[0].addChild(nodes[1]);

  var rootNode = nodes[0];
  checkMap(rootNode);
}
Tests that lf.tree.map() is constructing a new tree with the exact same
structure as the original tree, for a simple tree.