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 (#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
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
* 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
