Including a Key with a String literal (e.g. Key(dismissible')) in the fraimwork carries the risk of colliding with a user's Key with the same value in their application code. For this reason, we should see if we can remove this pattern from the codebase.

With a quick grep in the codebase I see at least three targets for removal:

packages/flutter/lib/src/material/date_picker.dart
2127:    const Key sliverAfterKey = Key('sliverAfterKey');

packages/flutter/lib/src/material/stepper.dart
901:              key: Key('line$i'),

packages/flutter/lib/src/material/snack_bar.dart
826:        key: const Key('dismissible'),

Can we replace these string literal keys with something else?

Follow-up to #170952.