function microSeconds() {
  var ret = Date.now() * 1000; // milliseconds to microseconds
  if (ret <= lastMs) {
    ret = lastMs + 1;
  }
  lastMs = ret;
  return ret;
}
Returns the current unix time in micro-seconds,
WARNING: This is a pseudo-function
Performance.now is not reliable in webworkers, so we just make sure to never return the same time.
This is enough in browsers, and this function will not be used in nodejs.
The main reason for this hack is to ensure that BroadcastChannel behaves equal to production when it is used in fast-running unit tests.