Let the second pass phantom callbacks run in a separate task on the foreground thread.

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 83bbd7e20f79933fea1539790e665dcfccf283da..3137ac2a9f66cc6ce154903030ea6a848f922d99 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -347,6 +347,7 @@ bool Shell::ExecuteString(Isolate* isolate, Handle<String> source,
       return false;
     }
     result = script->Run();
+    EmptyMessageQueues(isolate);
     data->realm_current_ = data->realm_switch_;
   }
   if (result.IsEmpty()) {
@@ -1985,6 +1986,11 @@ void Shell::CollectGarbage(Isolate* isolate) {
 }
 
 
+void Shell::EmptyMessageQueues(Isolate* isolate) {
+  while (v8::platform::PumpMessageLoop(g_platform, isolate)) continue;
+}
+
+
 #ifndef V8_SHARED
 bool Shell::SerializeValue(Isolate* isolate, Handle<Value> value,
                            const ObjectList& to_transfer,