Fix off-by-one error in AstTyper, part 2.

src/typing.h

 // Copyright 2013 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 65 matching lines @@
     e->set_bounds(Bounds::NarrowLower(e->bounds(), t, isolate_));
   }
 
   Effects EnterEffects() {
     store_ = store_.Push();
     return store_.Top();
   }
   void ExitEffects() { store_ = store_.Pop(); }
 
   int variable_index(Variable* var) {
+    // Stack locals have the range [0 .. l]
+    // Parameters have the range [-1 .. p]
+    // We map this to [-p-2 .. -1, 0 .. l]
     return var->IsStackLocal() ? var->index() :
-           var->IsParameter() ? -var->index() - 1 : kNoVar;
+           var->IsParameter() ? -var->index() - 2 : kNoVar;
   }
 
   void VisitDeclarations(ZoneList<Declaration*>* declarations);
   void VisitStatements(ZoneList<Statement*>* statements);
 
 #define DECLARE_VISIT(type) virtual void Visit##type(type* node);
   AST_NODE_LIST(DECLARE_VISIT)
 #undef DECLARE_VISIT
 
   DISALLOW_COPY_AND_ASSIGN(AstTyper);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_TYPING_H_