Support 'fvm dart' completion

src/_fvm

@@ -55,6 +55,14 @@ _fvm_run_flutter() {
   fi
 }
 
+_fvm_run_dart() {
+  local begin=$(($CURRENT - 1))
+  if (( $+functions[_dart] )); then
+    compset -n $begin
+    _dart "$@"
+  fi
+}
+
 _fvm() {
   typeset -A opt_args
   local context state line
@@ -93,6 +101,12 @@ _fvm() {
            '(- *)'{-h,--help}'[Print this usage information]' \
             && ret=0
           ;;
+        (dart)
+          _arguments -C \
+           '1: :_fvm_run_dart' \
+           '*: :_normal' \
+            && ret=0
+          ;;
         (exec)
           _arguments -C \
            '*::args:_normal' \