BottomSheetThemeData has an additional field modalElevation which makes it possible to set different elevations between persistent and modal bottom sheets.

* Add flag to use root navigator for showModalBottomSheet

* Add documentation

* Add tests

* Address feedback and fix analyzer issues

* Address feedback

* Update comments

Removed the GestureDetector from the modal bottom sheet that dismissed it on tap and updated several tests to accommodate this change.

* Introduce BottomSheetTheme and shape support for bottom sheet

* Add bottom sheet theme to ThemeData. Use theme in bottom sheet build

* Expose color, elevation, shape to showModalBottomSheet helper

* Expose color, elevation, shape to showBottomSheet helper

* Address PR feedback

* Address PR feedback

* Address additional PR feedback

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

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.

Also, extra docs.

Fixes https://github.com/flutter/flutter/issues/13279
Fixes https://github.com/flutter/flutter/issues/2115

This fixes the popup menu code to do a better job of expanding
smoothly regardless of which side of the screen it's on. It still
results in a bidirection growth when positioned at the bottom of the
screen, so maybe we'll need to animate menus differently, but that's
a problem for another patch.

Also, improve some docs and provide RelativeRect.toSize which I needed
at one point while building this patch (though it didn't survive all
the way to the end).

Some widgets are using closures even if the only values that are
captured are this, context or widget, that can be accessed even from
methods of the State object.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

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

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

* address review comments

The latter is the proper spelling.

Fixes #8883