Fix keyboard navigation in embedded DartPad

[atsansone]

Hotlist: https://buganizer.corp.google.com/hotlists/5891206
Bug: https://buganizer.corp.google.com/issues/338110557

Observed Results

The navigation using keyboard get stuck after entering the first box for executing commands.

• Screencast Voiceover
• Screencast ChromeVox

Expected Results

Every link/button should be accessible using the keyboard, the screen reader focus must reach all the controls in the page.

User Impact

Users with cognitive issue or that cannot rely in the use of mouse and use keyboard instead, will find a challenging time since the navigation cannot be continued, and they will not be able to see the rest of the page..

Steps To Reproduce

1. Enabled VoiceOver using Cmd + F5 or ChromeVox using Ctrl + Alt + Z.
2. Open https://dart.dev/codelabs/async-await
3. Start the linear navigation using Tab key.
4. Observe that when the focus reaches the first box for ¨Incorrectly using an asynchronous function¨, it is trapped and the navigation with keyboard cannot be continued.

Relevent GAR Sheet

A11Y Tracker.

GAR Details

2.1 Is the primary method of navigating and reaching content and controls accessible?, 1.3. Is focus always set correctly?, 1.2. Can all controls be reached and used via the keyboard without using a mouse?.

• Machine: macOS/ChromeBook
• Browser: Chrome Version 120.0.6099.272 (Official Build) (64-bit)
• Screen reader: VoiceOver/ChromeVox.
• Bug filling time: 15 mins.