Migrate to new flutter bootstrapping

[kenzieschmoll]

Warnings we are now seeing during build:

I took a stab at migrating this but it was not as straightforward as I would have hoped.

CC @eyebrowsoffire maybe the warning message can be tweaked to make the migration path more clear?

Bootstrapping changes: flutter/flutter#144434

[eyebrowsoffire]

BTW, we are planning for the warnings to link to the documentation once it is finalized. Issue tracking the link in the warning: flutter/flutter#145559

And the issue for documentation for the new bootstrapping in general: flutter/website#10126

I can assist you getting devtools unblocked if you need. The TL;DR is:

• Add a file called flutter_bootstrap.js in the web directory in your project, next to index.html
• Put this in flutter_bootstrap.js:

{{flutter_js}}
{{flutter_build_config}}
_flutter.loader.load({
  serviceWorkerSettings: {
    serviceWorkerVersion: {{flutter_service_worker_version}},
  },
  config: {
    canvasKitBaseUrl: 'canvaskit/',
  }
});

• Get rid of any flutter.js script tags in the index.html, and just put the following script tag somewhere:

<script src="flutter_bootstrap.js" async></script>

If you need help with it, just let me know, I'm happy to help iron out any issues.