This converts all remaining "## Sample code" segments into snippets, and fixes
the snippet generator to handle multiple snippets in the same dartdoc block
properly.

I also generated, compiled, and ran each of the existing application samples,
and fixed them up to be more useful and/or just run without errors.

This PR fixes these problems with examples:

1. Switching tabs in a snippet now works if there is more than one snippet in
   a single dartdoc block.
2. Generation of snippet code now works if there is more than one snippet.
3. Contrast of text and links in the code sample block has been improved to
   recommended levels.
4. Added five new snippet templates, including a "freeform" template to make
   it possible to show examples that need to change the app instantiation.
5. Fixed several examples to run properly, a couple by adding the "Scaffold"
   widget to the template, a couple by just fixing their code.
6. Fixed visual look of some of the samples when they run by placing many
   samples inside of a Scaffold.
7. In order to make it easier to run locally, changed the sample analyzer to
   remove the contents of the supplied temp directory before running, since
   having files that hang around is problematic (only a problem when running
   locally with the `--temp` argument).
8. Added a `SampleCheckerException` class, and handle sample checking
   exceptions more gracefully.
9. Deprecated the old "## Sample code" designation, and added enforcement for
   the deprecation.
10. Removed unnecessary `new` from templates (although they never appeared in
   the samples thanks to dartfmt, but still).

Fixes #26398
Fixes #27411

This makes the code more consistent but also fixes our flutter-analyze bot.

Issue https://github.com/dart-lang/sdk/issues/35940

This reverts commit 46cabdab.

https://github.com/flutter/engine/compare/d48de7a3ec97...3757390fa4b0

git log d48de7a3ec97ebf7ffe543aa74993efa51064789..3757390fa4b00d2d261bfdf5182d2e87c9113ff9 --no-merges --oneline
3757390fa Roll src/third_party/dart ecd7a88606..0a7dcf17eb (4 commits)
61d3080d5 Add FFI to libraries.yaml. (flutter/engine#7811)
f5259b8e2 Roll src/third_party/skia b6f53783337e..186669c4128b (8 commits) (flutter/engine#7816)
816921b9e Use newer Skia API for PathMeasure (flutter/engine#7809)
da56ff9c6 Roll src/third_party/skia 94a5328e0e4b..b6f53783337e (8 commits) (flutter/engine#7812)
04fbc2545 Roll src/third_party/skia 66f09a72995a..94a5328e0e4b (1 commits) (flutter/engine#7810)
347d690b2 Add support for new Scenic clip planes. (flutter/engine#7804)
309b90ca5 Roll src/third_party/skia 14d64dd4c47c..66f09a72995a (8 commits) (flutter/engine#7805)
1dead52a8 Roll src/third_party/skia 0b6ae6386d34..14d64dd4c47c (10 commits) (flutter/engine#7802)
74d94e52c Revert "Use all font managers to discover fonts for strut. (#7734)" (flutter/engine#7801)
769016c88 Roll src/third_party/skia 63d477cd99b0..0b6ae6386d34 (9 commits) (flutter/engine#7798)
f7eb3cb20 Roll src/third_party/skia 217acf58d0d8..63d477cd99b0 (1 commits) (flutter/engine#7797)
c4a55552b Use all font managers to discover fonts for strut. (flutter/engine#7734)
17b7d1e0a Roll src/third_party/dart 754e5f404c..ecd7a88606 (8 commits)
b4ed8cff3 Roll src/third_party/skia 233fc0b03c02..217acf58d0d8 (1 commits) (flutter/engine#7795)
27ab9f7af Roll src/third_party/skia 454e5fb7457d..233fc0b03c02 (4 commits) (flutter/engine#7794)
c58f5fefe Roll src/third_party/dart fdfe40ea95..754e5f404c (13 commits)

The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff (liyuqian@google.com), and stop
the roller if necessary.

This reverts commit d8cd2ff4.

This reverts commit 1d93e924.

In particular, make debugVisitOnstageChildren only work in debug mode, and add documentation.

This reverts commit 965dc52e as performance gains seems to outweigh compressed apk size increase.

* Fixes offset and unblocks roll

This reverts commit 76061c4f.

This PR aims at several things:

1. Use pub_semver to check a version in pubspec.yaml meets the requirements specified in https://semver.org/.
2. Don't limit build-number/build-name as a fixed format. Instead, validate it according to the target(ios/android).
3. Make sure that build-number/build-name are always validated no matter it's specified by the `flutter command` or version in pubspec.yaml.

Fixes #27589

* Make plugin path search logic use version check as a fallback.

* In JetBrainsToolbox, Info.plist would be just some kind of wrapper, we need a double check before get the real one.

* Use Regex to get content instead of `default command` to avoid error output as there could be no JetBrainsToolboxApp key/value for a normal Info.plist(Android Studio.)

When creating a flutter project,
Make sure: applicationid for android conforms to: https://developer.android.com/studio/build/application-id

Make sure: bundleid for ios conforms to:
https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/understanding_utis/understand_utis_conc/understand_utis_conc.html
(Not only the name,but also the organization, as it's a check for whole bundleid.)

This makes Android raw key event handling use the "character" data coming from the engine properly, and gets rid of the "toLowerCase" hack that I was using to normalize logical key events, in favor of using the new "plainCodePoint" that has the unmodified code point (the code point as if no modifier keys were pressed).

just a small typo 
the word "the constraints" is duplicated

* Update stepper.dart

Added 'physics' prop which allows the developer to assign the Stepper's scroll view's 'ScrollPhysics' which can solve with unwanted scrolling behaviour if the parent is also a scroll view. Defaults to 'AlwaysScrollableScrollPhysics' if null.

* Update stepper.dart

Removed unnecessary import and defaulting to 'AlwaysScrollableScrollPhysics' if physics prop is null.

* Update stepper.dart

Ran through format to remove unwanted whitespace which Analysis isn't happy about.

* Update stepper.dart

Tried reformatting again.

* Update stepper.dart

Tried reformatting again.

* Update stepper.dart

Formatting again why do you hate me github editor.

* Update stepper.dart

* Update stepper.dart

if this format doesn't work i'll cry

* Update stepper_test.dart

Added Stepper scroll tests. One that fails to find Text after Stepper if physics left as null and one that succeeds if physics set to `ClampingScrollPhysics()`

* Update stepper_test.dart

Added const constructors

* Update stepper_test.dart

trying to get rid of whitespace again

* Update stepper_test.dart

* Update stepper_test.dart

why whitespace why

* Update stepper_test.dart

* Update stepper_test.dart

* Update stepper_test.dart

* Update stepper_test.dart

Swapped to `findsNothing` because I'm an idiot.

* Update stepper_test.dart

* Update stepper.dart

[Material] Update the card demo in the Gallery to demonstrate different uses of the Card widget (#27699)

Additionally, this adds a tappable Card example to the Card documentation.

This pattern is problematic when Flutter is ported to build systems
that require each executable end-point to be self-contained (e.g.
Bazel).