Fix mjsunit/debug-script.js failure that occurs when running with the --isolates flag.

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index c6f2ead60155ed8996b86724b55fa3f238e45044..f20a41aa5fc7bb56a3feab434c46e717d0a0e44a 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -1589,6 +1589,11 @@ void SourceGroup::ExecuteInThread() {
         Execute();
       }
       context.Dispose();
+      if (Shell::options.send_idle_notification) {
+        const int kLongIdlePauseInMs = 1000;
+        V8::ContextDisposedNotification();
+        V8::IdleNotification(kLongIdlePauseInMs);
+      }
     }
     if (done_semaphore_ != NULL) done_semaphore_->Signal();
   } while (!Shell::options.last_run);