This PR contains a series of minor changes to address issues that I happened to run into:

 - Pretty-print errors triggered when handling events that are pending because of having locked event handling. Previously these were just dumped to the console.
 - Add more documentation for `debugPaintPadding`.
 - Add documentation to `Tween` saying how to implement it.
 - Slight formatting changes in the scrollbar code to align some expressions.
 - Since we convert ScrollMetricsNotifications to ScrollNotifications in various places, provide an explicit API to do this. This will make the behaviour consistent throughout, and makes the code easier to understand. Added a test for this.
 - Clarifications to some of the BindingBase and SchedulerBinding documentation.
 - Clarified some documentation in `flutter_test`'s `Finder` class.

Remove 1745 decorative breaks

Fix error message for unbounded viewports

* 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

* Check for invalid elevation usage in the layer tree

* 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.

In theory, this should aid with tree shaking and minutely optimize the code in release builds.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Diagnostics object is.

* Refactor SemanticsData and SemanticsNode to use Diagnosticable.

* Switch toStringDeep to take named parameters.

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

These properties had a high cost in the documentation because they're
all top-level properties, and there's really very little reason for
these values to be configurable in the first place.

Add Diagnosticable base class and documentation