This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

This re-lands 3c56e6c7 on top of a new Dart SDK that contains new mixin syntax fixes.

This reverts commit 3c56e6c7.

It breaks the AOT build.

* first pass

* revert WidgetsBindingObserver

* disable prefer_mixin

* docs and more mixins

* newer engine; binding fixes

* upgrade dependencies

* fix test binding

* remove whitespace

* remove the obsolete `enableSuperMixins: true`

* upgrade dartdoc to 0.22.0

* temporarily use git version of dartdoc

* fix pub global activate syntax

* use dartdoc 0.23.0

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

UnconstrainedBox will allow its child to size itself as if it had no constraints, and then attempt to fit around that object, until its own constraints are exceeded, in which case it will clip and display an overflow warning.

I also factored out DebugOverflowIndicator, which will draw overflow indicators on containers which overflow but aren't expected to.