Breaks the moveBy call from drag and dragFrom into two separate calls and changes the default behavior of DragStartBehavior to DragStartBehavior.start (#26438)

* Breaking change which sets the default DragStartBehavior to DragStartBehavior.start and changes WidgetTester.drag and WidgetTester.dragFrom by breaking the moveBy function call into multiple pieces.

* Expose "center" on CustomScrollView

* Also support anchor

* the onStart callback will report the location of the pointer where it wins the gesture arena by default instead of the pointer down location. Fixes all tests related to changing this default value.

This reverts commit 08538f91.

* Adds start behavior option to the drag gesture recognizer and makes it the default option when a drag gesture recognizer is created. Also fixes all the tests to work correctly with the new default behavior.

* Handle a missing ListView separator as an error

* Handle missing item, and errors in itemBuilder and separatorBuilder

* CR fixes and move error handling into sliver.dart to handle all ListView constructors

* Only show an error for null separatorBuilder value in debug mode

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

* Avoid abbreviations

* Sample code for AppBar.leading

* Add a test for OverflowBox/FractionallySizedBox

* Minor wording improvements in the text.

The words "note that" here don't really contribute to the flow.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

This is follow-up to https://github.com/flutter/flutter/pull/21143.

* Roll engine to 56139397e6ae1257bdabd1ef79ff7aca96c67d4.6

Changes since last roll:
```
56139397e Roll src/third_party/skia 7ba1d64f0706..5f0726b01019 (12 commits) (#6104)
47a1ce0e6 Allow embedders to set the root surface transformation. (#6085)
```

* Roll engine to f3ff83a5db71262d240aa5337a2a9a22c73c4749. (dart roll).

* Add const

* Add ignore analyzer prefer_const_constructors_in_immutables

- remove an //ignore that is no longer needed
- fix some intrinsic methods that used the API incorrectly (shouldn't affect correctness but should make things a tiny bit more efficient)
- add some asserts to help track down bugs quicker
- update a TODO to point to the currently relevant bug
- fix some indenting
- improve the naming of some privates to improve readability

Fix ListView dartdoc where a paragraph doesn't flow with the section heading and the other paragraphs in the section (#18430)

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

Let translucent Cupertino bars have its scaffold children automatically pad their heights - second try (#13440)

* Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

* review

* More docs and comments from #13317

Revert "Let translucent Cupertino bars have its scaffold children automatically pad their heights (#13194)" (#13317)

This reverts commit 5c4ffa13.

* Let lists automatically add sliver padding from media query. Translucent nav and tab bars leave behind media query paddings in scaffolds.

* tests

* const lint

* Rename base abstract class to generalized ObstructingPreferredSizeWidget

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* fix miscellaneous typos in dartdocs

* moar typos

Fixes #11930

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Fixes #11855

This patch includes:

 * SliverPadding
 * ScrollView
 * PageView

Fixes #11764
Fixes #11836

Add Diagnosticable base class and documentation