feat(goctl): api dart support flutter v2 (#1603)

0. support null-safety code gen
1. supports -legacy flag for legacy code gen
2. supports -hostname flag for server hostname
3. use dart official format
4. fix some some bugs

Resolves: #1602

tools/goctl/api/dartgen/format.go

@@ -0,0 +1,40 @@
+package dartgen
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+const dartExec = "dart"
+
+func formatDir(dir string) error {
+	ok, err := dirctoryExists(dir)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return fmt.Errorf("format failed, directory %q does not exist", dir)
+	}
+
+	_, err = exec.LookPath(dartExec)
+	if err != nil {
+		return err
+	}
+	cmd := exec.Command(dartExec, "format", dir)
+	cmd.Env = os.Environ()
+	cmd.Stderr = os.Stderr
+
+	return cmd.Run()
+}
+
+func dirctoryExists(dir string) (bool, error) {
+	_, err := os.Stat(dir)
+	if err == nil {
+		return true, nil
+	}
+	if os.IsNotExist(err) {
+		return false, nil
+	}
+	return false, err
+}