Enable iterating over roots from helper threads by moving stack walking.

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 "vm/code_observers.h"
@@ skipping 1580 matching lines @@
 
   // Visit objects in the megamorphic cache.
   megamorphic_cache_table()->VisitObjectPointers(visitor);
 
   // Visit objects in per isolate stubs.
   StubCode::VisitObjectPointers(visitor);
 
   // Visit objects in isolate specific handles area.
   reusable_handles_.VisitObjectPointers(visitor);
 
-  // Iterate over all the stack frames and visit objects on the stack.
-  StackFrameIterator frames_iterator(validate_frames);

[Ivan Posva, 2015/08/11 17:41:31]

Can you please explain how we are validating frames after this change?

-  StackFrame* frame = frames_iterator.NextFrame();
-  while (frame != NULL) {
-    frame->VisitObjectPointers(visitor);
-    frame = frames_iterator.NextFrame();
-  }
-
   // Visit the dart api state for all local and persistent handles.
   if (api_state() != NULL) {
     api_state()->VisitObjectPointers(visitor, visit_prologue_weak_handles);
   }
 
   // Visit the current tag which is stored in the isolate.
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&current_tag_));
 
   // Visit the default tag which is stored in the isolate.
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&default_tag_));
@@ skipping 549 matching lines @@
                            serialized_message_, serialized_message_len_);
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(I);
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart