function getSampleIndex(rowCount) {
  var index = new lf.index.RowId('dummyName');

  for (var i = 0; i < rowCount; ++i) {
    index.set(i, i * 100);
  }

  return index;
}
Creates a new Index pre-populated with dummy data to be used for tests.
@param {number} rowCount
@return {!lf.index.Index}