Migrate Dart sources from `dart:html` to `package:web`

[SleepySquash]

Synopsis

dart:html and package:js aren't compatible with WASM compilation.

Solution

Replace dart:html with package:web and package:js with dart:js_interop packages.

Things left to do:

• [x] migrate medea_flutter_webrtc to package:web
• [x] migrate _getName helper from js_util to js_interop
• [x] test whether everything works correctly

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains issue reference
  • [x] Has type and k:: labels applied
  • [x] Has assignee
• Before review:
  • [x] Documentation is updated (if required)
  • [x] Tests are updated (if required)
  • [x] Changes conform code style
  • [x] CHANGELOG entry is added (if required)
  • [x] FCM (final commit message) is posted or updated
  • [x] Draft mode is removed
• [x] Review is completed and changes are approved
  • [x] FCM (final commit message) is approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] All temporary labels are removed

[SleepySquash]

FCM

Migrate Dart sources from `dart:html` to `package:web` to support WASM (#178)

[SleepySquash]

@alexlapa, dartanalyze fails, cuz medea_flutter_webrtc needs to be updated first in order for the migration to properly work (medea_flutter_webrtc expects tracks to be from dart:html and not from the package:web). Not sure how this should be approached. Probably should first release medea_flutter_webrtc and only then proceed with this package.

Despite my tests, please, be sure to check both the web and native Flutter implementations still work.