Clear duration timeout after dismiss

Fixes #4859

Author: Sven Reglitzki

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

@@ -81,7 +81,10 @@ export class MdSnackBar {
     // If a dismiss timeout is provided, set up dismiss based on after the snackbar is opened.
     if (config.duration > 0) {
       snackBarRef.afterOpened().subscribe(() => {
-        setTimeout(() => snackBarRef.dismiss(), config.duration);
+        const timeoutHandle = setTimeout(() => snackBarRef.dismiss(), config.duration);
+        snackBarRef.afterDismissed().subscribe(() => {
+          clearTimeout(timeoutHandle);
+        });
       });
     }