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

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 VM_HEAP_H_
 #define VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 87 matching lines @@
   // Move external size from new to old space. Does not by itself trigger GC.
   void PromoteExternal(intptr_t size);
 
   // Heap contains the specified address.
   bool Contains(uword addr) const;
   bool NewContains(uword addr) const;
   bool OldContains(uword addr) const;
   bool CodeContains(uword addr) const;
   bool StubCodeContains(uword addr) const;
 
-  // Visit all pointers.
+  // Visit all pointers. Caller must ensure concurrent sweeper is not running,
+  // and the visitor must not allocate (see issue 21620).
   void VisitObjectPointers(ObjectPointerVisitor* visitor) const;
 
-  // Visit all objects.
+  // Visit all objects, including FreeListElement "objects". Caller must ensure
+  // concurrent sweeper is not running, and the visitor must not allocate (see
+  // issue 21620).
   void VisitObjects(ObjectVisitor* visitor) const;
 
   // Find an object by visiting all pointers in the specified heap space,
   // the 'visitor' is used to determine if an object is found or not.
   // The 'visitor' function should be set up to return true if the
   // object is found, traversal through the heap space stops at that
   // point.
   // The 'visitor' function should return false if the object is not found,
   // traversal through the heap space continues.
   // Returns null object if nothing is found. Must be called within a
@@ skipping 222 matching lines @@
   NoHeapGrowthControlScope();
   ~NoHeapGrowthControlScope();
  private:
   bool current_growth_controller_state_;
   DISALLOW_COPY_AND_ASSIGN(NoHeapGrowthControlScope);
 };
 
 }  // namespace dart
 
 #endif  // VM_HEAP_H_