Fix issue where SliverPersistentHeader that is both floating and pinned would scroll down when scrolling past the beginning of the ScrollView (#27433)

Fixes https://github.com/flutter/flutter/issues/25025

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 reverts commit 46cabdab.

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

Before this, we had several places where an isReleaseMode was defined, all with the same definition. This just makes it more broadly visible to allow our users to use it, as well as creating debug and profile versions, and adding a device lab test for it.

Since this is a const value, this makes it possible for a developer to easily mark blocks that can be removed at AOT compile time.

Integrate Strut: Add StrutStyle, expose Strut API, wire up strut with dart:ui, Roll engine 31a7f4d..e7eb1c8 (7 commits) (#26332)

Includes a breaking change to dart:ui ParagraphStyle where lineHeight is renamed to height for consistency with TextStyle.

This implements support for hovering mouse pointers, so that mice connected to Android devices, and ChromeOS devices running Android apps will work properly.

It teaches flutter_test about hover events, which required changing how they are created and used.

Also modifies AnnotatedRegion to allow a region that can be located someplace other than just the origin.

Along with tests for all of the above.

Fixes #5504

* Fixes cursor on iOS devices

* fix some bad indentations

* remove suspicious line

Fixes #20893, the most-upvoted bug for framework.

A detailed analysis explaining the cause of the bug has been posted at #20893 (comment).

Generally, we should always use default-less switches for
platform-specific behaviour so we're forced to make a conscious decision
on each instance of such behaviour any time we add support for a new
platform.

I fixed this TODO a while ago during my a11y work.

This was added to make sure we don't crash before the engine is rolled
with the system channel API addition and is no longer needed.

* adds force press select word functionality

flutter/engine#7307 changes the engine side of embedded UIView to only
reject gestures when the framework sends a `rejectGesture` message, so
that gesture resolution can done after a touch sequence has ended (see
PR description for flutter/engine#7307 for more details).

This change makes the framework send a `rejectGesture` message to the
engine when a UiKitView rejects a gesture.

I'm planning to land this PR before the engine side change, so right now
it swallows the exception thrown if there is no engine implementation
for `rejectGesture` (which keeps us with the current behavior). After
this change lands I'll land the engine PR, and then clean up the part
that swallows the exception.