Verify that appendChild(script) does not perform a microtask checkpoint

Source/core/html/parser/HTMLScriptRunner.cpp

 /*
  * Copyright (C) 2010 Google, Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 268 matching lines @@
         ScriptLoader* scriptLoader = toScriptLoaderIfPossible(script);
 
         // This contains both and ASSERTION and a null check since we should not
         // be getting into the case of a null script element, but seem to be from
         // time to time. The assertion is left in to help find those cases and
         // is being tracked by <https://bugs.webkit.org/show_bug.cgi?id=60559>.
         ASSERT(scriptLoader);
         if (!scriptLoader)
             return;
 
+        ASSERT(scriptLoader->isParserInserted());
+
         Microtask::performCheckpoint();
 
         InsertionPointRecord insertionPointRecord(m_host->inputStream());
         NestingLevelIncrementer nestingLevelIncrementer(m_scriptNestingLevel);
 
         scriptLoader->prepareScript(scriptStartPosition);
 
         if (!scriptLoader->willBeParserExecuted())
             return;
 
         if (scriptLoader->willExecuteWhenDocumentFinishedParsing()) {
             requestDeferredScript(script);
         } else if (scriptLoader->readyToBeParserExecuted()) {
             if (m_scriptNestingLevel == 1) {
                 m_parserBlockingScript.setElement(script);
                 m_parserBlockingScript.setStartingPosition(scriptStartPosition);
             } else {
                 ScriptSourceCode sourceCode(script->textContent(), documentURLForScriptExecution(m_document), scriptStartPosition);
                 scriptLoader->executeScript(sourceCode);
             }
         } else {
             requestParsingBlockingScript(script);
         }
     }
 }
 
 }