See https://github.com/flutter/flutter/issues/74381

This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.

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.

Added a check to the analyzer script to detect skipped tests that aren't commented.

The comment following the `skip` parameter should include either a link to a
github issue tracking the reenabling of the test, or a '[intended]' tag with
a brief description of why the test should never be enabled for the given
condition.

Python 2 has been end-of-life'd since January 2020, all Python scripts
on which Flutter depends have been migrated to Python 3. This changes
documentation and remaining invocations to explicitly invoke python3.

Issue: https://github.com/flutter/flutter/issues/83043

This fixes how the sample analyzer handles missing_identifier errors. It was looking at the wrong line, and missing an else clause, so it was silently allowing missing_identifier errors to pass.

In addition, this fixes the sample generation so that it uses the correct filename for the output files: it previously was looking for the first line that had a filename, which was meant to indicate a non-generated line. This change adds a new Line.generated constructor for generated lines, so that they can also have the correct filename associated with them.

- Update dwds and and the rest of the packages

Closes: https://github.com/flutter/flutter/issues/87100

* Randomize tests, exclude tests that fail with randomization.

* Disable some more tool tests

Fixes a small problem with the analyze.dart script that fixes it so that it can be invoked from any directory, not just the Flutter root.

* Upgrade all packages

In particular, bring in dwds with latest fixes for flakes.

Closes: https://github.com/flutter/flutter/issues/85043
Related: https://github.com/flutter/flutter/issues/85575

* Re-enable fixed tests

* Disable failing test due to DDS issue

The (new, not yet used) code gen for iOS was setting up a std::map from key codes to logical and physical key codes, but it was using uint32_t, which isn't big enough to hold the Flutter key codes.

Also, iOS needs to be able to filter out function keys, so I added a function key set.

* [web] move e2e tests from flutter/engine to flutter/flutter
* flutter update-packages --force-upgrade

Remove `//dev/bots/run_fuchsia_tests.sh` as there are no longer any references to it.

Migrating tests to cfv2 will be easier if we don't have to account for this script.

See https://fxbug.dev/79691.