fix(snackbar): snacksbars sometimes don't get removed

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

@@ -219,6 +219,23 @@ describe('MdSnackBar', () => {
       });
     });
   }));
+
+  it('should remove past snackbars when opening new snackbars', async(() => {
+    snackBar.open('First snackbar');
+    viewContainerFixture.detectChanges();
+
+    snackBar.open('Second snackbar');
+    viewContainerFixture.detectChanges();
+
+    viewContainerFixture.whenStable().then(() => {
+      snackBar.open('Third snackbar');
+      viewContainerFixture.detectChanges();
+
+      viewContainerFixture.whenStable().then(() => {
+        expect(overlayContainerElement.textContent.trim()).toBe('Third snackbar');
+      });
+    });
+  }));
 });
 
 @Directive({selector: 'dir-with-view-container'})

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

@@ -48,7 +48,10 @@ export class MdSnackBar {
 
     // When the snackbar is dismissed, clear the reference to it.
     snackBarRef.afterDismissed().subscribe(() => {
-      this._snackBarRef = null;
+      // Clear the snackbar ref if it hasn't already been replaced by a newer snackbar.
+      if (this._snackBarRef == snackBarRef) {
+        this._snackBarRef = null;
+      }
     });
 
     // If a snack bar is already in view, dismiss it and enter the new snack bar after exit