BotBuilder-MicrosoftTeams: Can't @mention a Team or Channel

When sending an Activity into a Channel, you can @mention a single user, but you cannot do the same for a Team or Channel. This would be very helpful functionality for starting conversations that should be seen by an entire team.

For mentioning a user (using C#), it’s as simple as this:
// there is a ChannelAccount object with the correct ID called userAccount activity.AddMentionToText(userAccount, MentionTextLocation.PrependText, "Firstname Lastname");

Ideally, it would be just as simple to mention a team, like this:
activity.AddMentionToText(teamAccount, MentionTextLocation.PrependText, "Team Name");