Keep zone cached in SnapshotReader to allow removing ZoneHandle(Isolate*) interface.

runtime/vm/isolate.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/base_isolate.h"
@@ skipping 879 matching lines @@
   Dart_Port parent_port() const { return parent_port_; }
   char* script_url() const { return script_url_; }
   char* package_root() const { return package_root_; }
   char* library_url() const { return library_url_; }
   char* class_name() const { return class_name_; }
   char* function_name() const { return function_name_; }
   bool is_spawn_uri() const { return library_url_ == NULL; }
   bool paused() const { return paused_; }
 
   RawObject* ResolveFunction();
-  RawInstance* BuildArgs();
-  RawInstance* BuildMessage();
+  RawInstance* BuildArgs(Zone* zone);
+  RawInstance* BuildMessage(Zone* zone);
   void Cleanup();
 
  private:
   Isolate* isolate_;
   Dart_Port parent_port_;
   char* script_url_;
   char* package_root_;
   char* library_url_;
   char* class_name_;
   char* function_name_;
   uint8_t* serialized_args_;
   intptr_t serialized_args_len_;
   uint8_t* serialized_message_;
   intptr_t serialized_message_len_;
   bool paused_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_