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

* implicit-casts:false in flutter/lib/src/widgets

* address review comments

* address review comment

* 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 additional properties callback in Inspector Serialization Delegate

* Rename _SerializationDelegate to InspectorSerializationDelegate and add test

* Fix indentation

* Remove trailing whitespace

* Handle case when addAdditionalPropertiesCallback returns null

* Improve docs and minor renames

* Improve docs

* Improve documentation

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

* Adds a new parameter bool opaque to MouseRegion
* The same to RenderMouseRegion and AnnotatedRegionLayer
* Add findAnnotations to Layer 

* add missing trailing commas

* add more missing trailing commas

* add more and more missing trailing commas

* Make inspector details subtree depth configurable.

Teach Layer and its implementations, RenderObject and its implementations, and PaintingContext to reuse engine layers. The idea is that a concrete RenderObject creates a Layer and holds on to it as long as it needs it (i.e. when it is composited, and the layer type does not change). In return, each Layer object holds on to an EngineLayer and reports it to the engine via addRetained and oldLayer. This allows the Web engine to reuse DOM elements across frames. Without it, each frame drops all previously rendered HTML and regenerates it from scratch.

* add an error count field to the Flutter.Error event

* review comments; more tests

* normalize comments

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

* more ui-as-code

* address review comments

Reland "Text inline widgets, TextSpan rework (#30069)" with improved backwards compatibility (#34051)

This reverts commit 14414f35.

This reverts commit 86862c1e.

* Merge in changes for inline widgets

* Make analyzer happy, fix tests

* Fix analyzer

* Add missing doc

* Docs on RichText

* Fix analyzer some more

* Remove whitespace at end of line

* Update goldens

* Text fixes

* Analyzer

Corects a bnuch of typeos throuhgout teh Fluter codebsae.

Made use of the `misspell` tool:
https://github.com/client9/misspell

This changes Listener to trigger enter/move/exit in all Listeners below the pointer, not just the leaf region (the first region hit). This is because we need to allow listeners to be nested so that, say, a widget that handles changing color on hover, but also is wrapped in a Tooltip (that handles hover) can trigger both actions, not just one.

To that end, I added a findAll to Layer, similar to the existing find method that was previously used. It returns an iterator over annotated layers which match the given data type.

Since the findAll is implemented as returning an Iterable (and is sync*), I re-implemented the find routines as just returning the first result from findAll, since that should be just as efficient, and would then prevent duplication in the implementation.

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

* some space formattings

* always use blocks in if-else if a block is used

* format spaces in for and while

* allow multiline if conditions

* fix missing space

* 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.