add Zone.root to get root zone directly

[JiaLiPassion]

add runOutsideOfCurrentZone method just like runOutsideAngular so we can run under rootZone or fork a new zone under rootZone.

Zone.current.fork({name: 'testZone'}).run(function() {
        Zone.runOutsideOfCurrentZone(() => {
          expect(Zone.current.name).toEqual('<root>');
        });
      });

The motivation is to run some async code inside the zoneSpec's callback. Without the runOutsideOfCurrentZone method, such async call will cause infinite loop.
for example:

Zone.current.fork({
  name: 'testZone', 
  onScheduleTask: function(delegate, currentZone, targetZone, task) {
     asyncLog(task);
     return delegate.scheduleTask(targetZone, task);
  }
});

asyncLog will cause infinite loop, now we can

Zone.current.fork({
  name: 'testZone', 
  onScheduleTask: function(delegate, currentZone, targetZone, task) {
     Zone.runOutsideCurrentZone(() => {
        asyncLog(task);
    });
     return delegate.scheduleTask(targetZone, task);
  }
});

[mhevery]

I am sorry, but I don't see the need for this API. I am fine with adding root property, but the rest seems just sugar.

My concern is that new APIs need to be maintained, and documented. Personally my philosophy is to just give the bare minimum and have the developer build on top of that.

[mhevery]

I am fine with this API, but I would rename it to root

[mhevery]

I think this API gives too much complexity to the user. Same can be achieved by:

Zone.root.fork(zoneSpec).run(....)

So I don't see the need to have it.

[JiaLiPassion]

@mhevery , Ok, got it,I have modified the PR, please review