[Isolates] Allow running multiple isolates in shell and use this in tests.

src/top.cc

 // Copyright 2006-2008 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 443 matching lines @@
     VMState state(EXTERNAL);
     result = callback(v8::Utils::ToLocal(receiver_handle),
                       index,
                       type,
                       v8::Utils::ToLocal(data));
   }
   return result;
 }
 
 
-const char* Isolate::kStackOverflowMessage =
+const char* const Isolate::kStackOverflowMessage =
   "Uncaught RangeError: Maximum call stack size exceeded";
 
 
 Failure* Isolate::StackOverflow() {
   HandleScope scope;
   Handle<String> key = Factory::stack_overflow_symbol();
   Handle<JSObject> boilerplate =
       Handle<JSObject>::cast(GetProperty(js_builtins_object(), key));
   Handle<Object> exception = Copy(boilerplate);
   // TODO(1240995): To avoid having to call JavaScript code to compute
@@ skipping 377 matching lines @@
   Isolate::Current()->break_access()->Lock();
 }
 
 
 ExecutionAccess::~ExecutionAccess() {
   Isolate::Current()->break_access()->Unlock();
 }
 
 
 } }  // namespace v8::internal