ObjectGraph: Establish a HeapIterationScope *before* making the heap writable, least the sweeper in…

runtime/vm/heap.h

 // Copyright (c) 2012, 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_HEAP_H_
 #define RUNTIME_VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 332 matching lines @@
   // This heap is in read-only mode: No allocation is allowed.
   bool read_only_;
 
   // GC on the heap is in progress.
   Monitor gc_in_progress_monitor_;
   bool gc_new_space_in_progress_;
   bool gc_old_space_in_progress_;
 
   friend class Become;       // VisitObjectPointers
   friend class Precompiler;  // VisitObjects
+  friend class ObjectGraph;  // VisitObjects
+  friend class Unmarker;     // VisitObjects
   friend class ServiceEvent;
   friend class PageSpace;             // VerifyGC
   friend class IsolateReloadContext;  // VisitObjects
 
   DISALLOW_COPY_AND_ASSIGN(Heap);
 };
 
 
 class HeapIterationScope : public StackResource {
  public:
-  HeapIterationScope();
+  explicit HeapIterationScope(bool writable = false);
   ~HeapIterationScope();
 
  private:
   NoSafepointScope no_safepoint_scope_;
   PageSpace* old_space_;
+  bool writable_;
 
   DISALLOW_COPY_AND_ASSIGN(HeapIterationScope);
 };
 
 
 class NoHeapGrowthControlScope : public StackResource {
  public:
   NoHeapGrowthControlScope();
   ~NoHeapGrowthControlScope();
 
  private:
   bool current_growth_controller_state_;
   DISALLOW_COPY_AND_ASSIGN(NoHeapGrowthControlScope);
 };
 
 
 // Note: During this scope, the code pages are non-executable.
 class WritableVMIsolateScope : StackResource {
  public:
   explicit WritableVMIsolateScope(Thread* thread);
   ~WritableVMIsolateScope();
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_HEAP_H_