Move the back button and drawer opening logic into the app bar.
Move the tap-status-bar-to-scroll-to-top logic to using
ScrollControllers. Provide a PrimaryScrollController and a `primary`
flag on scroll views.
Make it possible to track when a route becomes or stops being poppable.
Anywhere that accepted IconData now accepts either an Icon or an
ImageIcon.
Places that used to take an IconData in an `icon` argument, notably
IconButton and DrawerItem, now take a Widget in that slot. You can wrap
the value that used to be passed in in an Icon constructor to get the
same result.
Icon itself now takes the icon as a positional argument, for brevity.
ThemeData now has an iconTheme as well as a primaryIconTheme, the same
way it has had a textTheme and primaryTextTheme for a while.
IconTheme.of() always returns a value now (though that value itself may
have nulls in it). It defaults to the ThemeData.iconTheme.
IconThemeData.fallback() is a new method that returns an icon theme data
structure with all fields filled in.
IconTheme.merge() is a new constructor that takes a context and creates
a widget that mixes in the new values with the inherited values.
Most places that introduced an IconTheme widget now use IconTheme.merge.
IconThemeData.merge and IconThemeData.copyWith act in a way analogous to
the similarly-named members of TextStyle.
ImageIcon is introduced. It acts like Icon but takes an ImageProvider
instead of an IconData.
Also: Fix the analyzer to actually check the stocks app.
I was also going to implement sorting and emptying the cart but the
current data model doesn't make that easy, so I gave up on that. That's
why the TODOs are moved around though.
