[web] switch from .didGain/LoseAccessibilityFocus to .focus

[yjbanov]

This is a repeat of #53134, which was merged prematurely.

Warning

Only land this after:

• Reland: Request focus if accessibility focus is given to a Focus widget (#142942) flutter#149840 lands in the framework.
• You have personally manually tested the change together with the latest framework on all browsers.

Original PR description

Stop using SemanticsAction.didGain/LoseAccessibilityFocus on the web, start using SemanticsAction.focus. This is because on the web, a11y focus is not observable, only input focus is. Sending SemanticsAction.focus will guarantee that the framework move focus to the respective widget. There currently is no "unfocus" signal, because it seems to be already covered: either another widget gains focus, or an HTML DOM element outside the Flutter view does, both of which have their respective signals already.

More details in the discussion in the issue flutter/flutter#83809.

Fixes flutter/flutter#83809
Fixes flutter/flutter#148285
Fixes flutter/flutter#143337

[kevmoo]

@yjbanov the framework fix landed! 😄

[yjbanov]

@kevmoo Thanks for the ping. I've been testing this, and I think we'll need another framework fix before landing this.

[kevmoo]

@yjbanov – which framework fix are we waiting on?

[yjbanov]

@yjbanov – which framework fix are we waiting on?

flutter/flutter#150648. It's breaking g3. I'm working on a g3fix. But I'm still going to need to manually retest after landing the framework PR, before this can go in.

[jiahaog]

Reason for revert: Breaking google3 in b/350131288.

There is a test that does something like the following, to check if a radio button is selected.

      // Send a bunch of tabs to focus on the correct radio button
      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
      await tester.pump();
      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
      await tester.pump();
      await tester.sendKeyEvent(LogicalKeyboardKey.tab);
      await tester.pump();

      // Toggle the radio button with space
      await tester.sendKeyEvent(LogicalKeyboardKey.space);
      await tester.pump();

      final selectedRadio =
          tester.widget<Radio<bool>>(find.byType(Radio<bool>).at(1));
      expect(selectedRadio.value, isTrue);

After this commit, the above test fails. See the linked bug above for more details.

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