fix(snack-bar): prevent error when opening multiple snack bars in fast succession (#2392)

Fixes #2390.

Author: crisbeto

src/lib/snack-bar/snack-bar-container.ts

@@ -92,11 +92,9 @@ export class MdSnackBarContainer extends BasePortalHost implements OnDestroy {
   /** Handle end of animations, updating the state of the snackbar. */
   onAnimationEnd(event: AnimationTransitionEvent) {
     if (event.toState === 'void' || event.toState === 'complete') {
-      this._ngZone.run(() => {
-        this.onExit.next();
-        this.onExit.complete();
-      });
+      this._completeExit();
     }
+
     if (event.toState === 'visible') {
       this._ngZone.run(() => {
         this.onEnter.next();
@@ -131,8 +129,14 @@ export class MdSnackBarContainer extends BasePortalHost implements OnDestroy {
    * Makes sure the exit callbacks have been invoked when the element is destroyed.
    */
   ngOnDestroy() {
-    // Wait for the zone to settle before removing the element. Helps prevent
-    // errors where we end up removing an element which is in the middle of an animation.
+    this._completeExit();
+  }
+
+  /**
+   * Waits for the zone to settle before removing the element. Helps prevent
+   * errors where we end up removing an element which is in the middle of an animation.
+   */
+  private _completeExit() {
     this._ngZone.onMicrotaskEmpty.first().subscribe(() => {
       this.onExit.next();
       this.onExit.complete();