Fixes https://github.com/flutter/flutter/issues/132157

![](https://media.giphy.com/media/KY2dtJNlGPH08w41FN/giphy.gif)

Fixes https://github.com/flutter/devtools/issues/5931

With Multi View applications on the way, we need to be able to manage
the state of multiple Inspector widgets in a consistent way.

Previously each Widget inspector would manage the state of it's own
inspection. This made for a confusing and inconsistent experience when
clicking on the widget inspector of different views.

This PR changes the state management to the WidgetInspectorService
static instance so that all widget inspectors can share that state.

# Demo


https://github.com/flutter/flutter/assets/1386322/70fd18dc-5827-4dcd-8cb7-ef20e6221291

Make Element tree root generic

Changes our inspection behaviour so that it also sends a postEvent on the ToolEvent stream.

This reverts commit ec8693e8.

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

* Fix null safety error in inspector service extensions taking variable numbers of args.

Fixes https://github.com/flutter/flutter/issues/68627.
Also fix all inspector tests that do not rely on golden image functionality
so that they run with --enable-experiment=non-nullable. This verifies there
is test coverage to ensure there isn't a regression.

* Remove uses of dynamic.

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Add function to set structured error early

* Remove unused imports

* Save test handler in setUp

* Add utils file for shared test service

* Rename structured error functions

* Use setUpAll to save error handler and call initStructuredError from other init

* Rename widget inspector test service

* Remove debugging print statement

* Move error handling setting back to initServiceExtensions

* Rename structured error handler in test

* Namespace environment variable

* Rename test

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