up the latest Dart roll

- Resubmit https://github.com/flutter/flutter/pull/14392 to fix
compilation errors

This reverts commit 1c236d52.

Add meta tag @optionalTypeArgs to the mixin classes where we don't expect a generic type to be specified at the points where the mixin is applied. This suppresses the analyzer warnings.

in https://github.com/dart-lang/sdk/issues/31984

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments