PRs closed in this release of flutter/engine

From Fri Nov 29 19:41:00 2018 -0800 to Thu Feb 21 20:22:00 2019 -0800

6399 System Channels, Plugins, Dart Entrypoint, FlutterFragment (cla: yes)

6558 Retained rendering in Fuchsia PhysicalShapeLayer (cla: yes)

6719 Add onStart hook to FlutterFragmentActivity (cla: yes)

6805 [OR] Offset.fromDirection and Size.aspectRatio (cla: yes)

6879 Allow FlutterViewController to be released when not initialized with an engine (cla: yes)

6903 [H] Undeprecate BigInteger support, but document what it actually does. (cla: yes)