[vm, gc] Require a safepoint for heap iteration.

runtime/vm/safepoint.h

 // Copyright (c) 2016, 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.
 
 #ifndef RUNTIME_VM_SAFEPOINT_H_
 #define RUNTIME_VM_SAFEPOINT_H_
 
 #include "vm/globals.h"
 #include "vm/lockers.h"
 #include "vm/thread.h"
@@ skipping 73 matching lines @@
   // and also tracks the number of recursive safepoint operations on the
   // same thread.
   int32_t safepoint_operation_count_;
 
   // If a safepoint operation is currently in progress, this field contains
   // the thread that initiated the safepoint operation, otherwise it is NULL.
   Thread* owner_;
 
   friend class Isolate;
   friend class SafepointOperationScope;
+  friend class HeapIterationScope;
 };
 
 /*
  * Set of StackResource classes to track thread execution state transitions:
  *
  * kThreadInGenerated transitioning to
  *   ==> kThreadInVM:
  *       - set_execution_state(kThreadInVM).
  *       - block if safepoint is requested.
  *   ==> kThreadInNative:
@@ skipping 263 matching lines @@
   }
 
  private:
   uint32_t execution_state_;
   DISALLOW_COPY_AND_ASSIGN(TransitionToVM);
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_SAFEPOINT_H_