Link function contexts directly to the previous context.

src/contexts.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 148 matching lines @@
 //
 // We may be able to simplify the implementation:
 //
 // - We may be able to get rid of 'fcontext': We can always use the fact that
 //   previous == NULL for function contexts and so we can search for them. They
 //   are only needed when doing dynamic declarations, and the context chains
 //   tend to be very very short (depth of nesting of 'with' statements). At
 //   the moment we also use it in generated code for context slot accesses -
 //   and there we don't want a loop because of code bloat - but we may not
 //   need it there after all (see comment in codegen_*.cc).
-//
-// - If we cannot get rid of fcontext, consider making 'previous' never NULL
-//   except for the global context. This could simplify Context::Lookup.
 
 class Context: public FixedArray {
  public:
   // Conversions.
   static Context* cast(Object* context) {
     ASSERT(context->IsContext());
     return reinterpret_cast<Context*>(context);
   }
 
   // The default context slot layout; indices are FixedArray slot indices.
@@ skipping 211 matching lines @@
 #ifdef DEBUG
   // Bootstrapping-aware type checks.
   static bool IsBootstrappingOrContext(Object* object);
   static bool IsBootstrappingOrGlobalObject(Object* object);
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_CONTEXTS_H_