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)

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.

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.

* Built first tab

* Small additions

* started tab 3

* Need color arithmetics

* tab 2 built

* finalize

* lint and tests

* review

* Reapply docs after rebase

* use color.computeLuminance

* linter

* nit

Alignment will eventually replace FractionalOffset.

* is default

* hit target

* correct test name

* review note

Fixes #9757

* more widget const constructors

* prefer const constructors

* address review comments

Also switches to single quotes for consistency with surrounding code.

Aggressively apply the const lint.

The latter is the proper spelling.

Fixes #8883

* prefer const constructors

* fix double throw

* Add cupertino to gallery and add CupertinoButto

* Use single quotes

* Add disabled state

* Some review notes

* Make button animation more responsive and tweak timing

* Renamed things Cupertino

* Button with background, move cupertino demos, move material demos

* Move 2 level list too

* Refactor various demo route names

* Some review notes

* More reviews and add test

* Linter as

* Move private constant up