feat(flutter): use up-to-date zsh completion (#11062)

plugins/flutter/flutter.plugin.zsh

@@ -12,3 +12,18 @@ alias flrd="flutter run --debug"
 alias flrp="flutter run --profile"
 alias flrr="flutter run --release"
 alias flupgrd="flutter upgrade"
+
+# COMPLETION FUNCTION
+if (( ! $+commands[flutter] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `flutter`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_flutter" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _flutter
+  _comps[flutter]=_flutter
+fi
+
+flutter zsh-completion >| "$ZSH_CACHE_DIR/completions/_flutter" &|