Enable verbalization of each line in embedded DartPad

[atsansone]

From https://buganizer.corp.google.com/issues/337850112

Observed Results

Navigating through the interactive code example, I noticed that the number of the line is not reached and verbalized by SR.

• Screencast VoiceOver
• Screencast ChromeVox

Expected Results

Ensure that when the user is navigating in every line the number also should be notified.

User Impact

Screen reader and Cognitive users are confused because they are now aware about how many lines the code has.

Steps to Reproduce

1. Enabled VoiceOver using Command + F5 also ChromeVox using Ctrl + Alt + Z
2. Open the following page: https://docs.flutter.dev/cookbook/design/snackbars#interactive-example
3. Continue the linear navigation using the Tab and Arrow keys and reach the interactive example code.
4. Listen to the results inside the code of the line number.

GAR Assessment

Flutter Docs

GAR Details

1.2. Can all controls be reached and used via the keyboard without using a mouse?

Environment Details

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