This adds support for semantics traversal ordering.

It is a companion to flutter/flutter#14060, adding support for a sortIndex in the semantics data passed to the engine.

Addresses flutter/flutter#12187

Use @Available checks for iOS 11 features

Only implement `application:performActionForShortcutItem:completionHandler:` on iOS 9.0+. And fix some tests. (#4600)

... which caused trouble for a11y "select all".

This reverts commit 89b6d435.

Apparently the clang toolchain is still hitting issues. Will give this
another go after the next clang toolchain update.

Guard code that deals with iOS safe area insets behind an @Available
check.

This cleans up some old TODOs from before out clang toolchain supported
@Available.

Consolidates the asset path lookup logic from FlutterDartProject and FlutterMain().

* Lookup FTLAssetsPath when deducing assets path on ios

* WS

* Proper fix

* Another whitespace fix

* Allow embedder clients to provide a resource context. Closes flutter/flutter#14263.

See https://github.com/flutter/flutter/pull/14275 for framework side change.

Also includes some minor clean-ups for consistency. 

Required for https://github.com/flutter/flutter/issues/13469.

Fixes https://github.com/flutter/flutter/issues/14093

Fixes https://github.com/flutter/flutter/issues/13392.

* Support .packages option in frontend_server, bundle_path in main_mac.

This is needed to be able to run  in  mode.

* Remove byte store tests

* Remove unused import

* Fix formatting

* Enable flutter test argument passing on win and linux.

* Fix formatting

Fixes https://github.com/flutter/flutter/issues/14092

We have a use for this for the internal messaging plugin. Instead of rolling our own, it made sense to add it here especially since it has the mirror functionality (wrap).

Fixes https://github.com/flutter/flutter/issues/13896

* This adds rpc call to simply set asset path.

This is needed when doing hot reload to pick up updated assets.

* Move asset_directory fetch for after view_id. Clean up return. Fix formatting.

* Add SetAssetBundlePath methods implementations for mac and ios

* Fix mac mm

* Fix formatting

* Merge and use nullptr

* Apply transform to textures on android 

Add support for --strong option in the engine, create a strong mode version of the platform file (#4504)

* Add a --strong option to the front end server so we can use strong mode with preview-dart-2.

* Plumb the --strong option through the dart controller into the VM.

* - Build a strong version of platform.dill for use with the engine.
- Fix a strong mode static error in the assert statement

* Enable asserts when running debug version even in strong mode.

* Use the correct platform dill file for linking when doing the aot builds.

* Fix formatting issue.

This reverts commit 368380cf.

This breaks Flutter external_ui test.

* Apply transform to textures on android

This reverts commit eee4a1ed.

Original commit caused external_ui integration test failure on Android.