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/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']. 
+add this line to the end of out/host_debug_unopt/args.gn:
+```
+icu_use_data_file=false
+```
-### Prepare icu data (TODO: try to build it into the library directly!)
+finally run ninja:
+```
+ninja -C out/host_debug_unopt/ flutter/third_party/txt:txt_lib
+```
-move the built icu data file from $FLUTTER_ROOT/out/host_debug_unopt/icudtl.dat to the StreamingAssets path
+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'].
+
 
 ### Creat symbolic
 

engine/src/shell/platform/unity/darwin/macos/uiwidgets_panel.mm

   args.struct_size = sizeof(UIWidgetsProjectArgs);
   args.assets_path = streaming_assets_path;
   args.font_asset = settings;
-  //TODO: not support icu yet
-  
-  std::string icu_path = std::string(streaming_assets_path) + "/icudtl.dat";
-  args.icu_data_path = icu_path.c_str();
   
   args.command_line_argc = 0;
   args.command_line_argv = nullptr;