Restructure/fix thread pointer visiting; added some asserts.

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 "include/dart_native_api.h"
 #include "platform/assert.h"
 #include "platform/text_buffer.h"
@@ skipping 2586 matching lines @@
   // thread in the constructor/destructor of MonitorLocker,
   // so we create a MonitorLocker object which does not do any
   // no_safepoint_scope_depth increments/decrements.
   MonitorLocker ml(threads_lock(), false);
   if (is_mutator) {
     if (thread->sticky_error() != Error::null()) {
       ASSERT(sticky_error_ == Error::null());
       sticky_error_ = thread->sticky_error();
       thread->clear_sticky_error();
     }
+  } else {
+    ASSERT(thread->api_top_scope_ == NULL);
+    ASSERT(thread->zone_ == NULL);
   }
   if (!bypass_safepoint) {
     // Ensure that the thread reports itself as being at a safepoint.
     thread->EnterSafepoint();
   }
   OSThread* os_thread = thread->os_thread();
   ASSERT(os_thread != NULL);
   os_thread->DisableThreadInterrupts();
   os_thread->set_thread(NULL);
   OSThread::SetCurrent(os_thread);
@@ skipping 235 matching lines @@
 void IsolateSpawnState::DecrementSpawnCount() {
   ASSERT(spawn_count_monitor_ != NULL);
   ASSERT(spawn_count_ != NULL);
   MonitorLocker ml(spawn_count_monitor_);
   ASSERT(*spawn_count_ > 0);
   *spawn_count_ = *spawn_count_ - 1;
   ml.Notify();
 }
 
 }  // namespace dart