fix(dialog): fix focus trap sometimes not working.

Closes #6590

Author: ErinCoughlan

src/components/dialog/dialog.js

@@ -877,7 +877,9 @@ function MdDialogProvider($$interimElementProvider) {
 
       // When focus is about to move out of the dialog, we want to intercept it and redirect it
       // back to the dialog element.
-      var focusHandler = angular.bind(element, element.focus);
+      var focusHandler = function() {
+        element.focus();
+      };
       topFocusTrap.addEventListener('focus', focusHandler);
       bottomFocusTrap.addEventListener('focus', focusHandler);