Make formatting error message side-effect-free.

src/isolate.cc

 // 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 1120 matching lines @@
             stack_trace_object = Handle<JSArray>(JSArray::cast(stack_property));
           }
         }
         if (stack_trace_object.is_null()) {
           // Not an error object, we capture at throw site.
           stack_trace_object = CaptureCurrentStackTrace(
               stack_trace_for_uncaught_exceptions_frame_limit_,
               stack_trace_for_uncaught_exceptions_options_);
         }
       }
+      // Stringify custom error objects for the message object.
+      if (exception_handle->IsJSObject() && !IsErrorObject(exception_handle)) {
+        bool failed = false;
+        exception_handle = Execution::ToString(exception_handle, &failed);
+        if (failed) {
+          exception_handle = factory()->LookupAsciiSymbol("exception");
+        }
+      }
       Handle<Object> message_obj = MessageHandler::MakeMessageObject(
           "uncaught_exception",
           location,
           HandleVector<Object>(&exception_handle, 1),
           stack_trace,
           stack_trace_object);
       thread_local_top()->pending_message_obj_ = *message_obj;
       if (location != NULL) {
         thread_local_top()->pending_message_script_ = *location->script();
         thread_local_top()->pending_message_start_pos_ = location->start_pos();
@@ skipping 907 matching lines @@
 
 #ifdef DEBUG
 #define ISOLATE_FIELD_OFFSET(type, name, ignored)                       \
 const intptr_t Isolate::name##_debug_offset_ = OFFSET_OF(Isolate, name##_);
 ISOLATE_INIT_LIST(ISOLATE_FIELD_OFFSET)
 ISOLATE_INIT_ARRAY_LIST(ISOLATE_FIELD_OFFSET)
 #undef ISOLATE_FIELD_OFFSET
 #endif
 
 } }  // namespace v8::internal