Better single stepping in VM debugger

runtime/vm/isolate.cc

 // Copyright (c) 2013, 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.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 371 matching lines @@
       main_port_(0),
       heap_(NULL),
       object_store_(NULL),
       top_context_(Context::null()),
       top_exit_frame_info_(0),
       init_callback_data_(NULL),
       library_tag_handler_(NULL),
       api_state_(NULL),
       stub_code_(NULL),
       debugger_(NULL),
+      single_step_(false),
       simulator_(NULL),
       long_jump_base_(NULL),
       timer_list_(),
       deopt_id_(0),
       mutex_(new Mutex()),
       stack_limit_(0),
       saved_stack_limit_(0),
       message_handler_(NULL),
       spawn_data_(0),
       is_runnable_(false),
@@ skipping 791 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart