Applies horizontal and bottom safe area insets to the Shrine demo in the
Gallery. Top insets are not applied due to the presence of the
omnipresent sliver app bar. Specifically, this ensures that the grid
cards are inset inside the iPhone X notch in horizontal mode, and that
the bottom of the grid is positioned above the iOS home indicator.

Discovered during an a11y review of the gallery.

* Fix scrolling of multiple SliverLists and SliverGrids

* Update sliver_fixed_extent_list.dart

* Update sliver_grid.dart

* controller, position and test

* Make controllers swappable

* WIP

* Create a ListWheelScrollPhysics

* Created picker and gallery demo and testing now

* Works. Ready to document and test.

* Document and add tests. Make the scroll controller more generic.

* minor cleanup

* review

* review

* fix tests

* stop using TransformLayers for now

* Make FadeTransition more efficient

* Update fade_transition_test.dart

* Update proxy_box.dart

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

Fixes #13743 
Fixes #12379
Follow-up to #13745

Also adds an option to hide gestures introduced by `InkWell` and `InkResponse` from the semantics tree (see also `GestureDetector.excludeFromSemantics`).

Reformats a SafeArea introduced in #13853.

Applies horizontal safe area insets to the animation demo in the
Gallery. Specifically, this ensures the back button is positioned
consistently with iOS expectations and that that main image card in the
detail view respects safe area insets.

This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.

Applies horizontal safe area insets to the video demo in the Gallery.
This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.

Applies horizontal safe area insets to the Pesto demo in the Gallery.
This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.

Applies horizontal safe area insets to the Contacts demo in the Gallery.
This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.

Applies media padding (e.g. iPhone X safe area insets) to the Material Design demos in the Gallery.

Covers the following demos:
* Buttons (via the TabbedComponentDemoScaffold change)
* Cards
* Expansion panels
* Grid list
* Icons
* Images (via the TabbedComponentDemoScaffold change)
* Page Selector
* Progress Indicator
* Scrollable tabs
* Selection controls (via the TabbedComponentDemoScaffold change)
* Snack bar
* Tabs
* Text fields
* Tooltips

Fixes #13594

Adds safe areas around:
1. The body of the colour swatch view (tab 1)
2. The chat header (tab 2)
3. The chat bubbles (tab 2)
4. The 'Sign In' launcher button (tab 3)

Rather than use a Center widget, center the title using AppBar's
centerTitle property. This ensures the title is horizontally centred
with respect to the screen rather than centred in the space between the
leading and trailing app bar widgets, which are asymmetrical in Shrine.

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

* Make drawer demo header not scroll

* Add test for gallery drawer demo and fix user accounts drawer header overflow

* Video demo is unsupported on simulator

Display a message stating this.

* Address review

* Add spinner to loading video

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

This reverts commit 5c4ffa13.

Extracts Tab2ConversationRow class that consolidates widget tree for a
chat message in the Cupertino navagation chat demo. This refactoring
simplifies adding SafeArea support to handle iOS 11 safe areas in a
followup patch.

* 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

Fixes the remaining known issues with widgets supporting RTL.

This is in preparation for adding a second card with a demo for animated
icons.

Table refactor is just a cleanup (that also fixes an alignment bug).