Go to https://dartpad.dev/embed-flutter.html?id=468b77f8f1e6de58f6e3eee9582502ef then click the run button. A beachball will appear for a number of seconds. According to a modal dialogue on the page this is because we are parsing JS.
<rdar://problem/57402113>
*** This bug has been marked as a duplicate of bug 199866 ***