feat(zone): Allow escaping of auto-digest mechanism.

Closes #557

lib/core/zone.dart

@@ -29,16 +29,20 @@ class LongStackTrace {
  * A better zone API which implements onTurnDone.
  */
 class NgZone {
-  NgZone() {
-    _zone = async.Zone.current.fork(specification: new async.ZoneSpecification(
+  final async.Zone _outerZone;
+  async.Zone _zone;
+
+  NgZone()
+      : _outerZone = async.Zone.current
+  {
+    _zone = _outerZone.fork(specification: new async.ZoneSpecification(
         run: _onRun,
         runUnary: _onRunUnary,
         scheduleMicrotask: _onScheduleMicrotask,
         handleUncaughtError: _uncaughtError
     ));
   }
 
-  async.Zone _zone;
 
   List _asyncQueue = [];
   bool _errorThrownFromOnRun = false;
@@ -142,6 +146,22 @@ class NgZone {
    */
   run(body()) => _zone.run(body);
 
+  /**
+   * Allows one to escape the auto-digest mechanism of Angular.
+   *
+   *     myFunction(NgZone zone, Element element) {
+   *       element.onClick.listen(() {
+   *         // auto-digest will run after element click.
+   *       });
+   *       zone.runOutsideAngular(() {
+   *         element.onMouseMove.listen(() {
+   *           // auto-digest will NOT run after mouse move
+   *         });
+   *       });
+   *     }
+   */
+  runOutsideAngular(body()) => _outerZone.run(body);
+
   assertInTurn() {
     assert(_runningInTurn > 0 || _inFinishTurn);
   }

test/core/zone_spec.dart

@@ -46,6 +46,23 @@ main() => describe('zone', () {
     })));
 
 
+    it('should allow executing code outside the zone', inject(() {
+      var zone = new NgZone();
+      var outerZone = Zone.current;
+      var ngZone;
+      var outsideZone;
+      zone.run(() {
+        ngZone = Zone.current;
+        zone.runOutsideAngular(() {
+          outsideZone = Zone.current;
+        });
+      });
+
+      expect(outsideZone).toEqual(outerZone);
+      expect(ngZone.parent).toEqual((outerZone));
+    }));
+
+
     it('should rethrow exceptions from the onTurnDone and call onError when the zone is sync', () {
       zone.onTurnDone = () {
         throw ["fromOnTurnDone"];