This reverts commit afb206d3.

* Roll Skia revision to 342977ced701d06df2b3d2eedd8b64aeae1eb5c5

* Updated license files

android_tools contains a toolchain that is specific to each host platform

Fixes https://github.com/flutter/flutter/issues/8969

The script keeps state in the objects representing the repository files,
and it also maintains a cache of licenses.  This state should be freshly
recreated for each component so data from previous components does not
leak through.

Fixes https://github.com/flutter/flutter/issues/8809

Breaking change: removed facilities for JSON and string messaging from FlutterView/FlutterViewController, leaving only binary messaging there. All other use of flutter communication now goes through FlutterMessageChannel and FlutterMethodChannels. Retained use of String and JSON codecs for now.

Companion flutter PR: flutter/flutter#8837

* Remove sky_snapshot sources

* update licenses

Flutter channels now also supported on iOS. Use FlutterMessageChannel or FlutterMethodChannel with binary/string/json/standard codecs.

* Add podspec to Flutter iOS framework.

Flutter iOS apps are already pod-enabled, so we might as well turn the
engine framework into a pod, so we don't have to manually copy the
current Flutter.framework into the app directory on every build, but
rather let Cocoapods wire things up for us.

For that to happen, we need a pod spec for Flutter.framework. This spec
will not be published, but rather downloaded as part of the engine
artifacts, and the app's Podfile will have a local path dependency on
it.

* Update licenses_golden...

* Roll Dart to 890dff146ac4733018c059b36bc4d67c45da3232

* uddate licenses

* roll dart version to pickup fixes in gen_snapshot

* update licenses

See https://github.com/flutter/flutter/issues/8106