Refactor snapshot writer's forward list into a reusable class.

runtime/vm/raw_object.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_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/token.h"
@@ skipping 437 matching lines @@
     uword tags = ptr()->tags_;
     return ClassIdTag::decode(tags);
   }
 
   friend class Api;
   friend class Array;
   friend class Code;
   friend class FreeListElement;
   friend class GCMarker;
   friend class ExternalTypedData;
+  friend class ForwardList;
   friend class Heap;
   friend class HeapMapAsJSONVisitor;
   friend class ClassStatsVisitor;
   friend class MarkingVisitor;
   friend class Object;
   friend class RawExternalTypedData;
   friend class RawInstructions;
   friend class RawInstance;
   friend class RawTypedData;
   friend class Scavenger;
@@ skipping 1429 matching lines @@
   COMPILE_ASSERT(kTypedDataInt8ArrayViewCid == kTypedDataInt8ArrayCid + 14);
   COMPILE_ASSERT(kExternalTypedDataInt8ArrayCid ==
                  kTypedDataInt8ArrayViewCid + 15);
   COMPILE_ASSERT(kNullCid == kExternalTypedDataInt8ArrayCid + 14);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_