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.

Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

* Add examples for LinearProgressIndicator

* Add examples for CircularProgressIndicator

* Change name Linear  -> Circular

* Remove unnecessary changes

* Add images to docs

* Fix semi-colon bugs

* Fix trailing spaces

* Fix more semicolon bugs

* Remove imports in docs

* Remove error causing line breaks

* Add ticker providers

* Add more ticker providers

* Remove assets and combine samples

* Removed 6 samples

* Remove unnecessary animation
Co-authored-by: John Ryan <ryjohn@google.com>

* Remove unnecessary animation
Co-authored-by: John Ryan <ryjohn@google.com>

* Use material theme
Co-authored-by: John Ryan <ryjohn@google.com>

* Apply suggestions from code review
Co-authored-by: John Ryan <ryjohn@google.com>

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

* Fix even more unresolved doc references in material

* ++

* ++

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* implicit-casts:false in flutter/lib/src/material

* address review comments

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* Make progress indicators accessible

* Adds functionality for progress indicator height to be controlled by the parent.

* Improve documentation and clean up code.

* Remove "Note that".

The phrase "note that" is basically meaningless as a prefix to an
otherwise fine sentence.

Fixes https://github.com/flutter/flutter/issues/21445

* 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.