This reverts commit d35671c6.

* Fix FlutterError.onError in debug mode

* update

* fix comments

* add license header

* fix analyzer

* update

* another attempt

* fix test

* fix comment

This converts Diagnosticable to be a mixin instead of an abstract class, so that it can be used to add diagnostics to classes which already have a base class.

It leaves in place the DiagnosticableMixin mixin, since there are some plugins that are still using it, and removing it would mean that those plugins wouldn't work with master branch Flutter anymore. DiagnosticableMixin will be removed once this mixin version of Diagnosticable makes its way to the stable branch.

This reverts commit 46ccd086.

This PR fixes #49647 by changing DiagnosticableMixin.toDiagnosticsNode and other things to accept DiagnosticableMixin instead of requiring Diagnosticable.

This normalizes all of the "See also" blocks in comments so that they conform in style.

They all look like this now:

  /// See also:
  ///
  ///  * [MyFavoriteWidget], because it's cool.
  class Foo {}

I removed some useless "See also" blocks, and added commentary to ones that were just "bare" references.

This is my penance for adding so many bullets that are "-" instead of "*". :-)

* 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

No code changes, just comments: I spell-checked all the comments in the repo.

* add missing trailing commas

* add more missing trailing commas

* add more and more missing trailing commas

* shard tests

* make foundation tests pass

Flutter widget tests assert if a test completes with timers still
pending.  However, it can be hard to diagnose where a pending timer
came from.  For example, a widget might consume a third-party library
that internally uses a timer.

I added a FakeAsync.pendingTimersDebugInfo getter to quiver
(https://github.com/google/quiver-dart/pull/500).  Make flutter_test
use it.

Additionally modify Flutter's debugPrintStack to take an optional
StackTrace argument instead of always printing StackTrace.current.

Fixes #4237.

* [web][upstream] Fix debugPrintStack for web platform

Fixes: https://github.com/flutter/flutter/issues/37488

* use sentence case in error message titles

* add a test

Rendering errors with root causes in the widget layer should have a reference to the widget (#32511)

* more ui-as-code

* address review comments

Breaking change to extremely rarely used ParentDataWidget.debugDescribeInvalidAncestorChain api changing the return type of the method from String to DiagnosticsNode.

Skip diagnostics on release mode, add test in firebase testlab

Updates documentation and non-public API to use American spellings for
consistency with the rest of the codebase.

No changes to behaviour... other than how it's spelt.

Make FlutterError objects more structured so they can be displayed better in debugging tools such as Dart DevTools.