Fixes #11374

When scrolling vertically, the scrollbar is on the left in RTL.

* ensureVisible works with viewports that have a center child

* review feedback

Fixes #11369

Fixes #11358

Fixes #11389

Fixes https://github.com/flutter/flutter/issues/11817

* Exclude SemanticNodes that are fully covered by an overlapping sliver

* simplification

* formatting

* nits

* add test for center widget

This patch adds Positioned.directional and PositionedDirectional. The
former is useful when you already know the TextDirection. The latter is
useful when you want to use the ambeient Directionality.

Fixes #11390

* Roll engine forward

* Update engine dep one commit further

* Update engine dep one commit further

Extract a Snapshotter class that can be shared between FLX snapshotting,
AOT snapshotting, and assembly AOT snapshotting. Allows for better
testability of snapshotting logic.

* Extracts script snapshotting used in FLX build.
* Adds tests for snapshot checksumming, build invalidation/skipping.

Remaining work: disentangle + extract AOT snapshotting and Assembly AOT
snapshotting logic from build_aot.dart.

Since the called function can't throw, there is no need for the "guarded".
Since the function returns something, running in guarded mode doesn't really work. The `uncaught` handler wouldn't know what to return (except for `null`).

* Notify the completer after the close animation completes on the snackbar

* Review comments

* Fix tests and analyzer warnings

* Fix analyzer warnings

In some cases, the element tree is not clean but we are required to dump the tree anyway.
To avoid crashing in those cases, we return an explicit null node.

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

* Introduce a Directionality inherited widget which sets the ambient LTR vs RTL mode (defaulting to null, which means you cannot use directionality-influenced values).

* Make it possible to configure Padding (including Container.padding and Container.margin) using a directionality-agnostic EdgeInsets variant.

* Provide textDirection and verticalDirection controls on Row and Column to make them RTL-aware.

* Introduce a variant of FractionalOffset based on the EdgeInsets variant. Not yet actually used.

* Fix all the tests that depended on Row defaulting to LTR.

This change re-introduces skipping AOT snapshot builds if input sources
and outputs have not changed since the last snapshot build, assuming a
build for the same platform in the same build mode.

This reverts commit 3d5afb5a.
It includes the following changes relative to the original:
  1. Include the entrypoint source in the checksums
  2. include the build mode in the checksums
  3. include the target platform in the checksums

* refactor to ensureAction + some related doc fixes

* Update docs for markNeedsSemanticsUpdate

* rewording

* rewording

* ensureAction test

* ensureAction test

* ensureAction test

* more tests

* refactor to allways reset node

* tiny fixes

* more test

* doc fixes

* one more test

* review comments

Adds excludes for new test added in
13dda7cf (#11557)

This change ensures that snapshot build checksums used to avoid
duplicate builds are invalidated by a change to framework revision
(in case gen_snapshot is updated), as well as by build mode.

Currently, only FLX snapshotting uses checksums to avoid duplicate
builds. FLX snapshotting is always done with BuildMode.debug, so didn't
include build mode in the checksum file.

* Fix paintBounds/semanticsBounds of RenderSliver

* another test

* Properly update semantics when Material/Cupertino Slider are disabled

* whitespace fix

* fix style