Merge branch 'zxw/dev_mac_update_engine' of github.com:Unity-Technologies/com.unity.uiwidgets into zxw/dev_ios_update_engine

engine/README.md

 ```
 cd $FLUTTER_ROOT
 ./flutter/tools/gn --unoptimized
-ninja -C out/android_debug_unopt/ flutter/third_party/txt:txt_lib
+ninja -C out/host_debug_unopt/ flutter/third_party/txt:txt_lib
 ```
 
 If the compilation fails because "no available Mac SDK is found" (in flutter-1.17 the build tool will only try to find Mac 10.XX SDKs), please modify the file "/src/build/Mac/find_sdk.py" under flutter root by setting "sdks" as your current sdk, e.g., ['11.0']. 
 
 ```
 cd <uiwidigets_dir>\engine
-mono bee.exe
+mono bee.exe mac
 ```