Implement rewind: drop one or more frames from the debugger.

runtime/observatory/tests/service/step_over_await_test.dart

 // 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=--error_on_bad_type --error_on_bad_override  --verbose_debug --trace_service
+// VMOptions=--error_on_bad_type --error_on_bad_override  --verbose_debug
 
 import 'dart:async';
 import 'dart:developer';
 
 import 'test_helper.dart';
 import 'service_test_common.dart';
 
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 22 matching lines @@
   // Check that we are at the async statement
   (Isolate isolate) async {
     expect(M.isAtAsyncSuspension(isolate.pauseEvent), isTrue);
   },
   asyncStepOver,
   stoppedAtLine(LINE_B),
   resumeIsolate,
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);