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/util.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path"
 	"strings"
 
 	"github.com/zeromicro/go-zero/tools/goctl/api/spec"
@@ -34,6 +35,10 @@ func pathToFuncName(path string) string {
 	return util.ToLower(camel[:1]) + camel[1:]
 }
 
+func getBaseName(str string) string {
+	return path.Base(str)
+}
+
 func getPropertyFromMember(member spec.Member) string {
 	name, err := member.GetPropertyName()
 	if err != nil {