function nextUid() {
  return ++uid;
}
A consistent way of creating unique IDs in angular.

Using simple numbers allows us to generate 28.6 million unique ids per second for 10 years before
we hit number precision issues in JavaScript.

Math.pow(2,53) / 60 / 60 / 24 / 365 / 10 = 28.6M

@returns {number} an unique alpha-numeric string