Avoid using progress indicator within async builder in Flutter web

[daohoangson]

Closes #538

[github-actions]

Demo app

Deployed cdd4d02 to https://example-laygzswnr-flutter-widget-from-html.vercel.app.

Quick links:

• supported/tags.html
• Hello World
• Hello World (core)
• Wordpress
• --web-renderer canvaskit
• --web-renderer html

[mdeandrea-mrmilu]

I guess than this solve the error:

The following assertion was thrown while finalizing the widget tree:
Multiple widgets used the same GlobalKey.

The key [GlobalKey#48f84 ftnt_ref1] was used by multiple widgets. The parents of those widgets were:
- WidgetPlaceholder<String>(#ftnt_ref1, dependencies: [TshWidget])
- Listener(listeners: [down], behavior: deferToChild, renderObject: RenderPointerListener#8097d relayoutBoundary=up33)
A GlobalKey can only be specified on one widget at a time in the widget tree.

[daohoangson]

Looks like an unrelated bug @mdeandrea-mrmilu