This is motivated by part of the 2D scrolling proposal: [flutter.dev/go/2D-Foundation](https://flutter.dev/go/2D-Foundation)

This is one of the last little PRs to prep for the 2D scrolling foundation. 
This adds an optional `axis` parameter to the static Scrollable methods `[of, maybeOf, recommendDeferredLoadingForContext]`. This allows developers that are nesting scrollables (or one day using 2D scrolling) to look them up instead by a particular axis.

In general, even outside the context of 2D, I think this is helpful. I am often asked how to get the outer scrollable when nesting. Now it can be done.

There is also a small semantic refactor here in ScrollableState.build, this just creates a private method (_buildChrome) that will be overridden in 2D later. It is easier to add now than in the really big PR that will be.

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* 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

Introduced by merging an old PR (https://github.com/flutter/flutter/pull/21157).

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* make @immutable const

* fix build