Do not visit slots in the top-level code generator's backend....

src/fast-codegen.cc

 // Copyright 2009 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 172 matching lines @@
     FunctionBoilerplateLiteral* expr) {
   UNREACHABLE();
 }
 
 
 void FastCodeGenerator::VisitConditional(Conditional* expr) {
   UNREACHABLE();
 }
 
 
-void FastCodeGenerator::VisitVariableProxy(VariableProxy* expr) {
-  Comment cmnt(masm_, "[ VariableProxy");
-  Expression* rewrite = expr->var()->rewrite();
-  ASSERT(rewrite != NULL);
-
-  // Forward to the proxy's rewrite.
-  rewrite->set_location(expr->location());
-  Visit(rewrite);
+void FastCodeGenerator::VisitSlot(Slot* expr) {
+  // Slots do not appear directly in the AST.
+  UNREACHABLE();
 }
 
 
 void FastCodeGenerator::VisitRegExpLiteral(RegExpLiteral* expr) {
   UNREACHABLE();
 }
 
 
 void FastCodeGenerator::VisitObjectLiteral(ObjectLiteral* expr) {
   UNREACHABLE();
@@ skipping 54 matching lines @@
   UNREACHABLE();
 }
 
 
 void FastCodeGenerator::VisitThisFunction(ThisFunction* expr) {
   UNREACHABLE();
 }
 
 
 } }  // namespace v8::internal