As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

The new version has an API change. Also, up our SDK requirements to be a
bit more realistic.

Remove the unimplemented `more` option from the copy/paste toolbar.

Punting this feature for the near time, so I'm removing the dead option
from the toolbar.

- Handles appear with tap or long press.
- Toolbar appears with long press on text, or tap on handle.
- Correct toolbar items shown depending on context.

* Use mojo clipboard service for copy/paste toolbar.

BUG=https://github.com/flutter/flutter/issues/1567

* First pass at a material-style copy/paste toolbar.

This mimics the toolbar you see when selecting text in an Android
material app. There's still more to do (like integration with the system
clipboard), but this seemed like a good stopping point.

BUG=https://github.com/flutter/flutter/issues/1567