function assertHighlightedIndex(explanation, index) {
  assertEquals(explanation, index, container.getHighlightedIndex());
  for (let i = 0; i < container.getChildCount(); i++) {
    if (i == index) {
      assertTrue(
          'Child at highlighted index should be highlighted',
          container.getChildAt(i).isHighlighted());
    } else {
      assertFalse(
          'Only child at highlighted index should be highlighted',
          container.getChildAt(i).isHighlighted());
    }
  }
}
Checks that getHighlighted() returns the expected value and checks
that the child at this index is highlighted and other children are not.
@param {string} explanation Message indicating what is expected.
@param {number} index Expected return value of getHighlightedIndex().