Support BuiltValue in the inspector

May applications particularly in g3 use BuiltValue heavily in their widget code. BuiltValue objects are not expandable which is confusing as they look a lot like Diagnosticable values that are.

The toString representation for Diagnosticable is lso slightly confusingly inconsistent with the one used by Flutter.
Extend the Inspector and Debugger views in DevTools so that BuiltValue is always expandable.

Bonus: make BuiltValue editable by understanding the builder patters for built value.

Fyi @davidmorgan.
One option would be to support BuiltValue types that extend Diagnosticable. Another option is to put the logical all in DevTools.