Add a facility to show messages from flutter tools to users.

[devoncarew]

Add a facility for the flutter tool to send messages to IDEs for display to users. This would be used for flutter sdk 'upgrade available messages for instance. This might be plumbed in through the flutter daemon protocol.

cc @zanderso

[devoncarew]

Ah, we never removed the daemon event to handle this:

'daemon.showMessage'

https://github.com/flutter/flutter/blob/master/packages/flutter_tools/doc/daemon.md#daemonshowmessage

With fields:

    String level; /* error | warning | info */
    String title;
    String message;

I don't know that that's enough info to know how to display a crash to the user. We'd want to know to display it as a status line item, not a toast. And, we'd need to include some kind of information about the crash (a stack trace?) to make actionable.

What is the expected use case? These crashes would already have been reported automatically to crash reporting by the flutter tool?

[devoncarew]

If we choose to do this, I think we'd need to create a new daemon.crash event w/ crash specific fields, and be clear about why we're reporting the crashes to the IDEs (instead of just relying on crash reporting).