[DAP] Support for assigning an inspected widget to a variable

[elliette]

When using the Widget Inspector in Flutter DevTools, from the console you can assign the inspected widget to a variable and then perform expression evaluation on that variable:

(Note this was added by @polina-c in flutter/devtools#5542)

It would be nice to offer the same support when debugging from VS Code, etc. I don't see any way to assign an inspected widget to a variable:

FYI @DanTup

[DanTup]

Yeah, this would be great. I guess there are a few parts to this:

1. Keeping track of what's inspected (although it's not clear to me what inspected means here.. things selected in the Widget Inspector, or things sent to the "inspect()" call? I think the goal is to stop the inspector from calling "inspect()" so these things might over lap today, but not in the future)
2. Handling assignment to these variables (detecting the assignment in an evaluation expression, extracting the variable name and adjusting the evaluation expression accordingly)
3. Handling evaluation of these variables (sending the names and IDs back in scope of the evaluation)