test.py: Bugfix in browser_controller. Make sure we always complete even in case of errors

dart/tools/testing/dart/browser_controller.dart

Index: dart/tools/testing/dart/browser_controller.dart
diff --git a/dart/tools/testing/dart/browser_controller.dart b/dart/tools/testing/dart/browser_controller.dart
index c6e07e7cb70a00b0900484dd32cb989828e5aab9..ecb082d1d928b467e5fbdd1336da25dfe580d15b 100644
--- a/dart/tools/testing/dart/browser_controller.dart
+++ b/dart/tools/testing/dart/browser_controller.dart
@@ -147,10 +147,9 @@ abstract class Browser {
           if (_cleanup != null) {
             _cleanup();
           }
-          doneCompleter.complete(true);
         }).catchError((error) {
           _logEvent("Error closing browsers: $error");
-        });
+        }).whenComplete(() => doneCompleter.complete(true));
       });
       return true;
     }).catchError((error) {