报错信息：
Failed to precompile json_model:json_model:
an those of overridden method 'HttpHeaders.add'.
void add(String name, Object value) {
^
org-dartlang-sdk:///sdk/lib/_http/http.dart:694:8: Context: This is the overridden method ('add').
void add(String name, Object value,
^
/D:/flutter/flutter_windows_v1.7.8+hotfix.4-stable/flutter/.pub-cache/hosted/pub.flutter-io.cn/http_multi_server-2.1.0/lib/src/multi_headers.dart:129:8: Error: The method 'MultiHeaders.set' has fewer named arguments than those of overridden method 'HttpHeaders.set'.
void set(String name, Object value) {
^
org-dartlang-sdk:///sdk/lib/_http/http.dart:703:8: Context: This is the overridden method ('set').
void set(String name, Object value,
^
pub finished with exit code 1
解决方案：
flutter pub upgrade
