function $$TaskTrackerFactoryProvider() {
  this.$get = valueFn(function(log) { return new TaskTracker(log); });
}
! This is a private undocumented service !

@name $$taskTrackerFactory
@description
A function to create `TaskTracker` instances.

A `TaskTracker` can keep track of pending tasks (grouped by type) and can notify interested
parties when all pending tasks (or tasks of a specific type) have been completed.

@param {$log} log - A logger instance (such as `$log`). Used to log error during callback
    execution.

@this