fix(dialog): backdrop not being removed if it doesn't have transitions

Fixes the dialog's backdrop not being removed if it's transition have been disabled.

Fixes angular#1607.

src/lib/core/overlay/overlay-ref.ts

@@ -92,7 +92,9 @@ export class OverlayRef implements PortalHost {
 
     // Add class to fade-in the backdrop after one frame.
     requestAnimationFrame(() => {
-      this._backdropElement.classList.add('md-overlay-backdrop-showing');
+      if (this._backdropElement) {
+        this._backdropElement.classList.add('md-overlay-backdrop-showing');
+      }
     });
   }
 
@@ -101,18 +103,27 @@ export class OverlayRef implements PortalHost {
     let backdropToDetach = this._backdropElement;
 
     if (backdropToDetach) {
+      let onTransitionEnd = () => {
+        if (backdropToDetach) {
+          backdropToDetach.parentNode.removeChild(backdropToDetach);
+
+          // It is possible that a new portal has been attached to this overlay since we started
+          // removing the backdrop. If that is the case, only clear the backdrop reference if it
+          // is still the same instance that we started to remove.
+          if (this._backdropElement == backdropToDetach) {
+            this._backdropElement = null;
+          }
+        }
+      };
+
       backdropToDetach.classList.remove('md-overlay-backdrop-showing');
       backdropToDetach.classList.remove(this._state.backdropClass);
-      backdropToDetach.addEventListener('transitionend', () => {
-        backdropToDetach.parentNode.removeChild(backdropToDetach);
-
-        // It is possible that a new portal has been attached to this overlay since we started
-        // removing the backdrop. If that is the case, only clear the backdrop reference if it
-        // is still the same instance that we started to remove.
-        if (this._backdropElement == backdropToDetach) {
-          this._backdropElement = null;
-        }
-      });
+      backdropToDetach.addEventListener('transitionend', onTransitionEnd);
+
+      // If the backdrop doesn't have a transition, the `transitionend` event won't fire.
+      // In this case we make it unclickable and we try to remove it after a delay.
+      backdropToDetach.style.pointerEvents = 'none';
+      setTimeout(onTransitionEnd, 500);
     }
   }
 }