- Added `InlineWidgetContainerDefaults` for deduping inline widget code
- Added a helper function `WidgetSpan.extractFromInlineSpan` for extracting `WidgetSpan`s and automatically applying text scaling (at widget level)
- Removed `TextPainter.inlinePlaceholderScales`. I'm going to deprecate the `scale` argument in `TextPainter.addPlaceholder` next, as scaling is now done at the widget level.
- Added runtime check and comments to make sure nobody is extending `PlaceholderSpan` directly (unfortunately we can't remove `PlaceholderSpan`  without moving RenderEditable and RenderParagraph to the widgets library).

* migrates lib/foundation and lib/widgets

* migrates lib/painting and lib/material

* migrates lib/cupertino, lib/gestures, lib/services, lib/semantics and lib/rendering

* fixes failing tests

* migrates test/

* improves hashing of lists

* migrate painting to nullsafety

* address review comments

* Add punctuation for unterminated sentences in doc comments

* Addressing review nit

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* 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 reverts commit ddd31bce.

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

This reverts commit 14414f35.

This reverts commit 160a5680.

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