It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.

The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.

Applies horizontal and bottom safe area insets to the Shrine demo in the
Gallery. Top insets are not applied due to the presence of the
omnipresent sliver app bar. Specifically, this ensures that the grid
cards are inset inside the iPhone X notch in horizontal mode, and that
the bottom of the grid is positioned above the iOS home indicator.

Alignment will eventually replace FractionalOffset.

Fixes https://github.com/flutter/flutter/issues/9342

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Aggressively apply the const lint.

* add @required when there's an assert not null

* address review comments

This machinery is useful for arbitrary boxes (e.g., with FittedBox).

Fixes #6463

The new name follows the pattern of the name suggesting which layout
protocol the parent expects the child to speak.

Fixes #8664

This happens when, for instance, you go back then forward quickly.

Also, use CustomScrollView in Shrine and fix a bug with one-line grids
not painting properly due to their reporiting zero paintExtent.

This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

These futures complete when the route is popped off the navigator. This
generalizes and simplifies a mechanism already in place for dialogs and
menus.

Fixes #5283

Fixes #3208.

* Removed unnecessary Shrine hero logic

These now have sorter names to make the callers less verbose.

Some folks didn't realize these existed and asked us to add them. By
using them in examples, hopefully folks will discover them more easily.

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.