Updating SetListener to SetEventListener which comes with a new SetHitViewTester

gn check does not run a C preprocessor, so it cannot understand that a
particular include does not apply to a particular build configuration.

See https://github.com/flutter/flutter/issues/8929

Fixes https://github.com/flutter/flutter/issues/8115

* link from Color to Colors

* from review

This reverts commit 7932ba76.

The Android and iOS versions of this function had diverged. This patch
makes the iOS version match the Android version (and what the framework
expects).

Fixes https://github.com/flutter/flutter/issues/8878

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

Fixes buildbot error use of undeclared identifier 'asl_log_message',
which I'm unable to repro locally.

* Perform all iOS logging through ASL

As of iOS 10, ASL is deprecated and replaced with os_log. ASL calls
continue to result in logging but as of iOS 10.3, only ASL_LOG_NOTICE
level and above are logged.

This change partially reverts 2937f06a,
adding back stdout and stderr redirection, which resulted in loss of
some direct writes to stdout that were necessary for debugging.

This change replaces the direct use of syslog with ASL on iOS, which
Apple has stated will continue to log on iOS >= 10. This eliminates the
need for the previous fwd-declaration of syslog.

See https://github.com/flutter/flutter/issues/8635

ASL is deprecated and replaced by os_log() on iOS. As of iOS 10.3,
calling this function breaks our logging altogether. os_log isn't
available pre-iOS 10.0. Rather than implement version checks and
conditional logic, this change eliminates the existing redirection
altogether. All engine code should be logging via the syslog redirection
implemented in Logger_PrintString in dart_runtime_hooks.cc.

Since stdio redirection is eliminated, we eliminate the flag that
controls whether such redirection is enabled.

See https://github.com/flutter/flutter/issues/7561

This property will eventually replace OffsetBase.isInfinite.

See https://github.com/flutter/flutter/issues/4301

* Remove sky_snapshot sources

* update licenses

Names matter, and I apparently forgot to test after adding ' Engine' to
the name. Names can't contain spaces, and must match the filename.

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