This is a follow-up to #162093.

The following material widgets should use the SemanticsRole.radioGroup as automatically as possible:

• Material Radio
• Material RadioListTile (the .adaptive constructor should cover CupertinoRadio as well; more discussion in [a11y] apply radioGroup role to cupertino widgets #164589)
• Material SegmentedButton
• Material time picker

It may not be possible for some widgets to automatically create a group. For example, a Radio or RadioListTile alone does not form a group. However, when they are in a group, they should participate seamlessly.

Widgets that have a clear intent to form a group of radio widgets, should apply the radioGroup role automatically, such as SegmentedButton (when used in exclusive mode), and showTimePicker that contains groups of exclusive radios (e.g. "AM"/"PM").

See also the Cupertino version of this issue: #164589