add some code lint in Future.then

[HxBreak]

#当使用链式调用时增加代码提示

Dio dio = Dio();
dio.get("http://www.baidu.com/").then((v){
print(v.data);
})

之前的代码会显示为动态类型没有成员提示

[codecov-io]

Codecov Report

Merging #11 into master will increase coverage by 0.73%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage    85.8%   86.53%   +0.73%     
==========================================
  Files           9        9              
  Lines         317      312       -5     
==========================================
- Hits          272      270       -2     
+ Misses         45       42       -3

Impacted Files	Coverage Δ
lib/src/Dio.dart	86.13% <77.77%> (+1.11%)	⬆️

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