This reverts commit cc1cf13e.

* Let CupertinoTabScaffold's tab be programmatically selectable

* Re-use the tab bar's index instead

* review

This adjusts the CupertinoTabBar height in the presence of bottom
padding. On the iPhone X, this will increase the height to account for
the home indicator widget.

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.

* 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

* Refactor CupertinoScaffold

* Rename rootTabPageBuilder to tabBuilder

* fix tab transparency padding

* add a CupertinoTab

* Add default background color

* Add a bunch of examples

* A bunch of tests

* Refactor CupertinoScaffold

* Rename rootTabPageBuilder to tabBuilder

* fix tab transparency padding

* Add default background color

* review notes

* fix test

* review

* Rename CupertinoTab to CupertinoTabView

* remove final ; in sample code for analyzer

* works using pageview
it’s a bit heavy and scrolls
no documentation or test yet

* docs and tests

* more docs tweak

* revert drive by page view changes

* swap out the indexed stack for a custom thing

* layout everything on the cheap
tests pass

* add more tests

* move back to stack and add docs

* review notes

* build all the tabs once built

Create a CupertinoNavigationBar without automatic adding of the lead button

Move some Cupertino colors to a common file. 

Create a CupertinoTabBar widget to mimic iOS looks