Reland of VariableProxy: when cloning, don't even think about creating dangling references.

src/ast/ast.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/ast/ast.h"
 
 #include <cmath>  // For isfinite.
 
 #include "src/ast/compile-time-value.h"
 #include "src/ast/prettyprinter.h"
@@ skipping 168 matching lines @@
       next_unresolved_(nullptr) {
   bit_field_ |= IsThisField::encode(variable_kind == THIS_VARIABLE) |
                 IsAssignedField::encode(false) | IsResolvedField::encode(false);
 }
 
 VariableProxy::VariableProxy(const VariableProxy* copy_from)
     : Expression(copy_from->position(), kVariableProxy),
       end_position_(copy_from->end_position_),
       next_unresolved_(nullptr) {
   bit_field_ = copy_from->bit_field_;
-  if (copy_from->is_resolved()) {
-    var_ = copy_from->var_;
-  } else {
-    raw_name_ = copy_from->raw_name_;
-  }
+  DCHECK(!copy_from->is_resolved());
+  raw_name_ = copy_from->raw_name_;
 }
 
 void VariableProxy::BindTo(Variable* var) {
   DCHECK((is_this() && var->is_this()) || raw_name() == var->raw_name());
   set_var(var);
   set_is_resolved();
   var->set_is_used();
 }
 
 
@@ skipping 760 matching lines @@
 // static
 bool Literal::Match(void* literal1, void* literal2) {
   const AstValue* x = static_cast<Literal*>(literal1)->raw_value();
   const AstValue* y = static_cast<Literal*>(literal2)->raw_value();
   return (x->IsString() && y->IsString() && x->AsString() == y->AsString()) ||
          (x->IsNumber() && y->IsNumber() && x->AsNumber() == y->AsNumber());
 }
 
 }  // namespace internal
 }  // namespace v8