Previously we called onPageChanged when the scroll ended, but that is too late.
Now we call onPageChanged when we cross the halfway mark, which, for example,
makes the tab indicator update earlier.

Fixes #8265

When resizing a PageView from 0x0, we weren't sure what the old page
number was because all the pages are collapsed at zero. Now we avoid the
divide by zero and default to the initialPage.

Fixes #8285

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Fixes https://github.com/flutter/flutter/issues/8017

Also, rename ScrollableMetrics to ScrollMetrics, which follows the
naming convention for most of the other classes (e.g., ScrollPosition,
ScrollPhysics).

Finally, fix a bug whereby SingleChildScrollView could not have a
GlobalKey, because, write test, find bug.

We've decided not to store the scroll position in PageStorage because
routes now maintainState by default.

Fixes #8051

Tabs are now fully driven by slivers.

This patch improves PageView to the point where we can use it in the date
picker. Specifically, you now get onPageChanged notifications and you can
control which page is visible using a PageController.