* Fix snapshot fingerprinting in --preview-dart-2 mode.

This is a follow up to PR #14775 - instead of treating dill file
as an input treat as intermediate file and don't fingerprint it.

Make sure to always use original main Dart file as an entry
point for fingerprint calculation.

This fixes an issue that AOT snapshot would not be recompiled in
the preview-dart-2 mode if entry point changes, e.g.

$ flutter build aot -t t/x.dart --preview-dart-2
$ flutter build aot -t t/y.dart --preview-dart-2

The second invocation would not build AOT snapshot.

(This issue is visible on the microbencmarks bot)

It was failing in the case of an AppContext being in scope
but not having a `BotDetector` seeded in it.

* Fix SliverPadding dropping the chain on scrollOffsetCorrection propagation

* review doc

* Fix focus switching when changing tabs in CupertinoTabScaffold

* review

* Test focuses are saved within tabs

* review

* add a FAB NotchMaker to ScaffoldGeometry

* add a notchMaker to FloatingActionButton

* Initial implementation of BottomAppBar

Mainly includes the notch making logic.
Not yet tested as currently there is no way to make the FAB and the BAB
overlap, once #14368 to lands we could add unit tests to the
BottomAppBar as well.

* use a closeable for clearing the FAB notchmaker

* Fix packages tests when run off bots

* Tweak dependencies to make bots happy

* Review comment

https://github.com/flutter/flutter/issues/14519

https://github.com/flutter/flutter/issues/14798

Fixes https://github.com/flutter/flutter/issues/14751

* Fix snapshot fingerprint check for --preview-dart-2.

* Remove printTraces

These URLs all automatically redirect HTTP to HTTPS so makes sense to point users at the secure versions.

This will allow other tools to call into the roll_dev script
to know what version is about to be published without actually
publishing.

https://github.com/flutter/flutter/issues/14751

* fix potential call on null

* review

This is needed to support running with local engine's dart sdk.

This is follow-up to https://github.com/flutter/flutter/pull/14702.

* All lists produced by JSON parsing are List<dynamic>. If more
speficic type is required then they need to be explicitly cast, e.g.
using castFrom helper;
* Function of type (ByText) -> Finder is not a subtype of
(SerializableFinder) -> Finder because ByText is in the contravariant
position;
* In Dart 2 typed(any) should be used instead of any in mockito based
tests.

* Added sample code to AnimatedOpacity docs