Do not eagerly convert exception to string when creating a message object

src/debug.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/v8.h"
 
 #include "src/api.h"
 #include "src/arguments.h"
 #include "src/bootstrapper.h"
 #include "src/code-stubs.h"
@@ skipping 611 matching lines @@
 
   MaybeHandle<Object> maybe_exception;
   MaybeHandle<Object> result = Execution::TryCall(
       function, handle(context->global_proxy()), 0, NULL, &maybe_exception);
 
   // Check for caught exceptions.
   if (result.is_null()) {
     DCHECK(!isolate->has_pending_exception());
     MessageLocation computed_location;
     isolate->ComputeLocation(&computed_location);
-    Handle<Object> message = MessageHandler::MakeMessageObject(
+    Handle<JSMessageObject> message = MessageHandler::MakeMessageObject(
         isolate, MessageTemplate::kDebuggerLoading, &computed_location,
         isolate->factory()->undefined_value(), Handle<JSArray>());
     DCHECK(!isolate->has_pending_exception());
     Handle<Object> exception;
     if (maybe_exception.ToHandle(&exception)) {
       isolate->set_pending_exception(*exception);
       MessageHandler::ReportMessage(isolate, NULL, message);
       isolate->clear_pending_exception();
     }
     return false;
@@ skipping 2719 matching lines @@
   logger_->DebugEvent("Put", message.text());
 }
 
 
 void LockingCommandMessageQueue::Clear() {
   base::LockGuard<base::Mutex> lock_guard(&mutex_);
   queue_.Clear();
 }
 
 } }  // namespace v8::internal