Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.

This allows the two videos in the demo to play simultaneously
(matching the behavior from before the recent upgrade to the
video player plugin)

This reverts commit 7ba775a9.

Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.

Navigator 2.0: Refactor the imperative api to continue working in the new navigation system (#44930)

* 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

* Video Demo instrumentation

* Video Demo instrumentation

* Updated per review

* Fixed a typo

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* 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.

When setState() calls occur asynchronously, it's possible that the
Futures on which they're waiting don't complete until the widget is
offscreen. To avoid this, we check the widget is mounted before calling
setState().

I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.

No documentation or code changes here, other than changing doc image URLs.

Changes since last roll:
```
ed303c628 Roll dart sdk again. Previous roll required 23ae4fa098 revert. (#4966)
8cd272733 Revert "Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)" (#4965)
9199b40f2 Revert "Support multiple shells in a single process. (#4932)" (#4964)
6baff4c82 Support multiple shells in a single process. (#4932)
31c5bb427 Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)
c8e4c6984 Avoid copying the contents of large platform message responses (#4947)
5ff527295 Update to use new vulkan GrBackendRenderTarget ctor. (#4962)
0c8993a1a Update to use new vulkan GrBackendRenderTarget ctor (part 2) (#4963)
132ebdda8 Revert "Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)" (#4961)
11882ab9e Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)
```

Add consts