function assertHighlightedRange(rangeHighlight, startValue, endValue) {
  const rangeStr = `[${startValue}, ${endValue}]`;
  const rangeStart = 10 + 10 * startValue;
  assertEquals(
      `Range highlight for ${rangeStr} should start at ${rangeStart}` +
          'px.',
      rangeStart, rangeHighlight.offsetLeft);
  const rangeSize = 10 * (endValue - startValue);
  assertEquals(
      `Range highlight for ${rangeStr} should have size ${rangeSize}` +
          'px.',
      rangeSize, rangeHighlight.offsetWidth);
}
Verifies that rangeHighlight position and size are correct for the given
startValue and endValue. Assumes slider has default min/max values [0, 100],
width of 1020px, and thumb widths of 20px, with rangeHighlight drawn from
the centers of the thumbs.
@param {number} rangeHighlight The range highlight.
@param {number} startValue The start value.
@param {number} endValue The end value.
@suppress {missingProperties} suppression added to enable type checking