We were trying to cram too much functionality in to the Dialog widget. Now we
have AlertDialog and SimpleDialog to cover to two different kinds of dialogs in
the spec.
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.
We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.
Fixes#287
