Roll engine a50f1ef56a05..4e9e69d2b392 (4 commits) (#48394)

https://github.com/flutter/engine/compare/a50f1ef56a05...4e9e69d2b392

git log a50f1ef56a05..4e9e69d2b392 --first-parent --oneline
2020-01-08 skia-flutter-autoroll@skia.org Roll fuchsia/sdk/core/mac-amd64 from nbeLe... to gL-XG... (flutter/engine#15253)
2020-01-08 stuartmorgan@google.com Add plugin_registrar_windows.h to published wrapper (flutter/engine#15265)
2020-01-08 skia-flutter-autoroll@skia.org Roll src/third_party/dart bebc7d3af5bd..c0ca187f2699 (37 commits) (flutter/engine#15252)
2020-01-08 skia-flutter-autoroll@skia.org Roll fuchsia/sdk/core/linux-amd64 from xYmDF... to oNKzr... (flutter/engine#15257)


If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC garyq@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

bin/internal/engine.version

-a50f1ef56a05999bfa97a777cca14fd7a00e8454
+4e9e69d2b3920832da47d667ffcf1c402e25c96a