fix #778.

In normal case, ZoneTask can only transit to running state from scheduled state.
But in some complex cases, ZoneTask will try to run after being canceled.

var createInterface = require('readline').createInterface;
var Observable = require('rx').Observable;
require('zone.js');

var rl = createInterface({ terminal: true, input: process.stdin, output: process.stdout });
var keypress = Observable.fromEvent(rl.input, 'keypress');
var line = Observable.fromEvent(rl, 'line');
keypress.takeUntil(line).toPromise();

The case above describe such issue.

Be simplified, a target have 2 listeners for 1 event, and when the event trigger, the 1st event listener remove the 2nd, and then 2nd listener run will cause this issue.

The detailed process is:

1. createInterface and Observable.fromEvent('keypress') will create two eventListeners of keypress event on ReadableStream target.
2. keypress (fromEvent) also subscribe line Observable.
3. when user enter key in command line.
   • keypress eventlisteners (2 listeners) will begin to run.
   • listeners[0] createInterface listener https://github.com/nodejs/node/blob/master/lib/readline.js#L187 will run normally, and trigger line Observable
   • line Observable will then trigger keypress Observable
   • keypress Observable will auto dispose and being removed from ReadableStream, so the Task is canceled
   • now ReadableStream only have one listener, but in memory listeners from the beginning still have 2
   • listeners[1] begin to run, but it has been canceled, and error occurs.

It is a little tricky, I tried without zone.js, the removed listener will also run again but do nothing (in this case, the observable has been stopped).

So in this PR, I just check whether ZoneTask is notScheduled (canceled) or not before runTask, if it is the case, just return and do nothing.