Fix --compile_all flag (--compile-all is wrong)

runtime/observatory/tests/service/dominator_tree_test.dart

Index: runtime/observatory/tests/service/dominator_tree_test.dart
diff --git a/runtime/observatory/tests/service/dominator_tree_test.dart b/runtime/observatory/tests/service/dominator_tree_test.dart
index c5f62e8e367812fb690cb23f7b5e9cbefbc3dda2..ef4fcd87f79183184936bcf883de8072331ab3d7 100644
--- a/runtime/observatory/tests/service/dominator_tree_test.dart
+++ b/runtime/observatory/tests/service/dominator_tree_test.dart
@@ -1,7 +1,7 @@
 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
-// VMOptions=--compile-all --error_on_bad_type --error_on_bad_override
+// VMOptions=--compile_all --error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';