function assertFunctionReports(conversionFunction) {
  let reported = false;
  try {
    legacyconversions.setReportCallback(() => {
      reported = true;
    });
    conversionFunction('irrelevant');
    assertTrue('Expected legacy conversion to be reported.', reported);
  } finally {
    legacyconversions.setReportCallback(() => {});
  }
}
Asserts that conversionFunction calls the report callback.
@param {function(string) : *} conversionFunction