* Bump min SDK to 2.19.0-0

* fix

* Roll Flutter Engine from 67254d6e4b03 to 8d83b98c55b3

* Roll Dart SDK from 35a9facce191 to e517487c5679 (Dart 3.0) (#38105)

* Bump SDK versions.

* Bump Dart SDK version constraints

* Update shrine package to 2.0.1 (null safe version)

* Fix more tests.

* Include patches from Jason for min android sdk version

* Fix analyzer warning

* Add the channel paramter to the Dartpad samples

* Add sanity check test

* Make sample_id more generic

use 'dart pub' instead of calling the pub executable directly

This reverts commit 5fd259be.

Switch document generation to use the snippets package instead of the snippets code in the Flutter repo. In the process, some bugs in sample code analysis have been fixed, and I've fixed some more errors in the samples.

This will allow the snippets package to be developed separately from the Flutter repo, and reduce the code in the Flutter repo.

The snippets code is deleted in this PR.

I also converted some comments in the snippet templates to be regular comments instead of doc comments, because having a doc comment block before the imports causes the Dart import sorter to lose the comment. They should have been regular comments in the first place.

The snippets package resides in the assets-for-api-docs repo.

The sample analysis has also been converted to be run in parallel, and I've bumped the Dartdoc version to 1.0.2.

Add a check that a pubspec file exists before trying to read it.