Make it clearer that PreParser doesn't depend on Isolate.

src/parser.h

 // Copyright 2012 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 423 matching lines @@
     // For constructing objects returned by the traversing functions.
     typedef AstNodeFactory<AstConstructionVisitor> Factory;
   };
 
   explicit ParserTraits(Parser* parser) : parser_(parser) {}
 
   // Custom operations executed when FunctionStates are created and destructed.
   template<typename FunctionState>
   static void SetUpFunctionState(FunctionState* function_state, Zone* zone) {
     Isolate* isolate = zone->isolate();
-    function_state->isolate_ = isolate;
     function_state->saved_ast_node_id_ = isolate->ast_node_id();
     isolate->set_ast_node_id(BailoutId::FirstUsable().ToInt());
   }
 
   template<typename FunctionState>
-  static void TearDownFunctionState(FunctionState* function_state) {
+  static void TearDownFunctionState(FunctionState* function_state, Zone* zone) {
     if (function_state->outer_function_state_ != NULL) {
-      function_state->isolate_->set_ast_node_id(
-          function_state->saved_ast_node_id_);
+      zone->isolate()->set_ast_node_id(function_state->saved_ast_node_id_);
     }
   }
 
   // Helper functions for recursive descent.
   bool IsEvalOrArguments(Handle<String> identifier) const;
 
   // Returns true if the expression is of type "this.foo".
   static bool IsThisProperty(Expression* expression);
 
   static bool IsIdentifier(Expression* expression);
@@ skipping 375 matching lines @@
  private:
   static const int kLiteralTypeSlot = 0;
   static const int kElementsSlot = 1;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(CompileTimeValue);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_PARSER_H_