* Minor improvements to framework documentation.

* maybePop documentation fix

* Mark some flutter_test arguments required.

Without these set, we get a crash.

* Minor correction to ListView docs

Fixes the hasScrollBody flag not accounting for child size. Adds the ability to specify over-scroll behavior.

* WIP

* Fixing nested scroll view tests

* Fixing nested scroll view tests

* Fixing nested scroll view tests

* WIP

* Added flag for SliverFillRemaining

* Clean up

* Review

* Reverted nested scroll view tests

* Updating tests

* Updated for overscroll behavior

Add a more meaningful message to the assertion on children property of MultiChildRenderObjectWidget and SliverChildListDelegate. (#32487)

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

This reverts commit 77ab0b83.

Credit to @ymback

* fix some formatting issues

* address review comments

* fix indent

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

This reverts commit 1d93e924.

In particular, make debugVisitOnstageChildren only work in debug mode, and add documentation.

* sliver separation and test

* Handle a missing ListView separator as an error

* Handle missing item, and errors in itemBuilder and separatorBuilder

* CR fixes and move error handling into sliver.dart to handle all ListView constructors

* Only show an error for null separatorBuilder value in debug mode

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

* Improve documentation and clean up code.

* Remove "Note that".

The phrase "note that" is basically meaningless as a prefix to an
otherwise fine sentence.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Ensure that the _childElements map is properly traversed as a sparse list and not inflated with garbage collected children.

* Add tests to ensure Lists/Tabs with KeepAlive children can make large jumps, don't lose children (including after rebuild).

* lint unnecessary_new on samples

* fix tests

Fixes #14590.

* Fix scrolling of multiple SliverLists and SliverGrids

* Update sliver_fixed_extent_list.dart

* Update sliver_grid.dart

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.

* fix miscellaneous typos in dartdocs

* moar typos

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._