Assert no allocation during heap iteration; fix unsafe iteration in ObjectGraph.

runtime/vm/object_graph.cc

 // Copyright (c) 2014, 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/object_graph.h"
 
 #include "vm/dart.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
@@ skipping 129 matching lines @@
  public:
   explicit Unmarker(Isolate* isolate) : ObjectVisitor(isolate) { }
 
   void VisitObject(RawObject* obj) {
     if (obj->IsMarked()) {
       obj->ClearMarkBit();
     }
   }
 
   static void UnmarkAll(Isolate* isolate) {
+    PageSpace* old_space = isolate->heap()->old_space();
+    MonitorLocker ml(old_space->tasks_lock());
+    while (old_space->tasks() > 0) {
+      ml.Wait();
+    }
     Unmarker unmarker(isolate);
     isolate->heap()->VisitObjects(&unmarker);
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Unmarker);
 };
 
 
 ObjectGraph::ObjectGraph(Isolate* isolate)
@@ skipping 297 matching lines @@
   {
     WritePointerVisitor ptr_writer(isolate(), stream);
     isolate()->VisitObjectPointers(&ptr_writer, false, false);
   }
   stream->WriteUnsigned(0);
   IterateObjects(&visitor);
   return visitor.count() + 1;  // + root
 }
 
 }  // namespace dart