up the latest Dart roll

- Resubmit https://github.com/flutter/flutter/pull/14392 to fix
compilation errors

If compiler failed to produce Kernel binary then compile(...) returns
null to the caller. If we don't check for null we end up trying to
run file called "null" which causes a very confusing crash of the
flutter_tester.

* Use already downloaded dart sdk for frontend_server.

This is follow-up to https://github.com/flutter/flutter/pull/14610

* Remove reference to engine dart-sdk from artifacts

* Remove unused imports

* WIP to switch back to solution where dart sdk is downloaded into bin/cache/dart-sdk, but it is 'flutter precache -a' that can download all dart sdks for other platforms under bin/cache/artifacts/engine/<platform>/dart-sdk

* Download all dart sdks when 'flutter precache -a'

* add explanation about navigator to showDialog doc

* update doc by review

* have flutter analyze support --preview-dart-2

* review comments

* Initial version, seems to work

* Unit test for android license checker

* Cleanups

* Windows analyzer wants const.

* Refinements to timeout

* review comments

* Forgot a nit

Inlcudes:
* libtxt: specifically request the Roboto font in tests and benchmarks (flutter/engine#4675)
* Make Paragraph methods really return List<int> if they are declared as such (flutter/engine#4676)
* Stop using ClassHierarchy.deprecated_incremental (flutter/engine#4678)
* On iOS, render to an offscreen surface to avoid renderbuffer slowdown (flutter/engine#4680)

* Put generated kernel files into <build> directory

* USe default value for kernel file

Includes:
* Roll Skia to f802e757646f424b563fdc1f5b5ee3c677659e98 (flutter/engine#4677)

Pull code out of flx.dart:assemble() to make flx.dart:buildAssets
which creates the AssetBundle. This will allow us to create just
this instead of an entire FLX.

* Use engine-built dart sdk

* Download dart-sdk from engine

* Move up deps to fix dart sdk constraint problem

* Update update_dart_sdk.ps1 for Windows

* Fix tests so they pass analysis

* More types for tests

* Roll engine

* Update dart sdk stamp location in flutter.bat

* Add newline

Re-apply "Send nextId in addition to previousId for traversal order (#14676)"

Fixes #14688

* Let the gallery bundle Id ephemerally change to io.flutter.demo.gallery before deploying

* add a comment

Work around xcodebuild's undocumented freeze when -showBuildSettings is used with -allowProvisioning* (#14684)

Adds a ScaffoldGeometry class and ValueNotifier for it.
A scaffold's ScaffoldGeometry notifier is held in the _ScaffoldState, and is passed to _ScaffoldScope.
New ScaffoldGemometry objects are built and published to the notifier.

* Revert "Roll engine to 834d6eb02988e7f2c44eda05cec908378a1a9138 (#14664)"

This reverts commit c19fbe86.
Reason: timeouts while doing hot reload in hot_mode_dev_cycle__preview_dart_2_benchmark.

fixes.

Contains
* Roll Dart to 223eeb2ebe112aaaddca206aab55cd54b4e54391 (https://github.com/flutter/engine/pull/4659)

This is a temporary workaround until we solve #14594.

* Revert "Revert "Reduce xcodebuild noise #2" (#14641)"

This reverts commit 2d47481f.

* Stop scrapping xcodebuild output, get the right build settings

* clone the command params first

* Change the default for pub tracing to false for non-bot runs

* Review feedback: update docs

* Review feedback

This sets things up so that when we next roll a dev build to beta, it updates the docs.
This means that for a while (until we publish a beta) the dev docs will not be updating.

This adds geometric sort ordering back in for semantics nodes that don't have a sort order defined.

With this change, widgets that either have no sort order, or have an equivalent sort order, will be compared geometrically. The comparison is between the two starting corners, so it is TextDirection-aware: parent nodes that are set to have LTR text will compare upper left corners of their children, and upper right when set to RTL.

Also fixed a bug in the Transform widget that didn't mark modified nodes as needing a semantics update.