'protoc-gen-dart' is not recognized as an internal or external command, operable program or batch file. --dart_out: protoc-gen-dart:

$ protoc -Iproto proto/[file].proto --plugin=protoc-gen-dart=c:\Users\[youruser]\AppData\Roaming\Pub\Cache\bin\protoc-gen-dart.bat --dart_out=grpc:lib/generated