Use SDK version of BytesBuilder (dart-lang/web_socket_channel#130)

The `BytesBuilder` class was previously available only through `dart:io`
which prevented it's use in cross platform code. Dart version `2.10.0`
makes this class available from either `dart:io` or `dart:typed_data`.

Delete this copy to reduce the amount of code that needs to be migrated
to null safety.