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 #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');
+      }
     });
   }
 
@@ -101,9 +103,8 @@ export class OverlayRef implements PortalHost {
     let backdropToDetach = this._backdropElement;
 
     if (backdropToDetach) {
-      backdropToDetach.classList.remove('md-overlay-backdrop-showing');
-      backdropToDetach.classList.remove(this._state.backdropClass);
-      backdropToDetach.addEventListener('transitionend', () => {
+      let styles = getComputedStyle(backdropToDetach);
+      let onTransitionEnd = () => {
         backdropToDetach.parentNode.removeChild(backdropToDetach);
 
         // It is possible that a new portal has been attached to this overlay since we started
@@ -112,7 +113,16 @@ export class OverlayRef implements PortalHost {
         if (this._backdropElement == backdropToDetach) {
           this._backdropElement = null;
         }
-      });
+      };
+
+      backdropToDetach.classList.remove('md-overlay-backdrop-showing');
+      backdropToDetach.classList.remove(this._state.backdropClass);
+
+      if (parseFloat(styles.transitionDuration)) {
+        backdropToDetach.addEventListener('transitionend', onTransitionEnd);
+      } else {
+        onTransitionEnd();
+      }
     }
   }
 }