* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

Adds the ability for the iOS text selection menu to handle items that are too wide for the screen.

Adds an overflow menu to the text selection menu on Android when the items overflow.

* 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

This pattern is problematic when Flutter is ported to build systems
that require each executable end-point to be self-contained (e.g.
Bazel).