Allows one to use a different core snapshot without a custom engine build by just packaging a different one in the APK.

This fixes a runtime error triggered when calling `invokeRpc`:
```
type 'Future<dynamic>' is not a subtype of type 'Future<Map<String, dynamic>>'
```

Also adds a log message for why connections are failing, as well as a
default timeout when attempting to connect to a websocket.

Fixes https://github.com/flutter/flutter/issues/17597

Related to:

#14822
Dart-Code/Dart-Code#490
#13379

* pass semantics children in traversal and hit test orders

* explain why we are inverting _children

* Roll engine

(contains roll of Dart to version
97b6c2e09d6532eeb622d85f23109f3f3b6d2b61 )

(contains skia roll to 037be4e032ef31d7bc0cf914c972af97c456b17f)

* Patch to fix builds with custom targets on iOS simulator. #16787

* Fix for unused import.

* Code review changes applied.

Scan folders when asset specified finishes with `/`

This optimizes the AnimatedSwitcher so that it tags the right widget with its keyed subtree, and avoids rebuilding the transition unnecessarily.

This significantly improves the performance of Chips (which uses AnimatedSwitcher to swap out it's avatar and delete icon children).

This is a follow up on issue #17169 and the pull request #17298

This pull request adds the onChangeStart and onChangeEnd callbacks for CupertinoSlider. These are called when a user starts and ends a change respectively.

Pushing for @dcaraujo0872, the PR author.

This allows test environments other than `flutter test` to have a hook
into the test exception reporting. Some test environments, for example,
don't just dump error details to the console, but rather require them
to be reported to a separate server.

There's no reason for them to read/write.

Adds fuchsia_remote_debug_protocol gn file.

This contains the following commits:

flutter/engine@1179c38 Clear the raster cache result when bypassing raster cache.
flutter/engine@ea9f96b Roll src/third_party/skia/ dd6e62e28..44890ef00 (1 commit)
flutter/engine@5c4201f Roll src/third_party/skia/ 51371a433..dd6e62e28 (4 commits; 1 trivial rolls)
flutter/engine@a56f2f2 Roll src/third_party/skia/ 000e00df1..51371a433 (11 commits; 1 trivial rolls)

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.