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

Source/core/html/parser/HTMLScriptRunner.cpp

Index: Source/core/html/parser/HTMLScriptRunner.cpp
diff --git a/Source/core/html/parser/HTMLScriptRunner.cpp b/Source/core/html/parser/HTMLScriptRunner.cpp
index 1efcad2a0241b1de84fd00c71227a54f12af545b..535d621752ff6d7c547c49b5d9cb458dda8de535 100644
--- a/Source/core/html/parser/HTMLScriptRunner.cpp
+++ b/Source/core/html/parser/HTMLScriptRunner.cpp
@@ -286,6 +286,8 @@ void HTMLScriptRunner::runScript(Element* script, const TextPosition& scriptStar
         if (!scriptLoader)
             return;
 
+        ASSERT(scriptLoader->isParserInserted());
+
         Microtask::performCheckpoint();
 
         InsertionPointRecord insertionPointRecord(m_host->inputStream());