Fix lint

[vancexu]

been there for too long....

[coveralls]

Coverage decreased (-0.4%) to 66.786% when pulling e585254 on lint into 74c2327 on master.

[vancexu]

It turns out to be my local lint is not using latest version, and the complains is removed from latest lint.
Using following to update lint and other tools.

export GO111MODULE=off                                                                                                                         
go get -u -v github.com/ramya-rao-a/go-outline                                                                                                 
go get -u -v github.com/acroca/go-symbols                                                                                                      
go get -u -v github.com/nsf/gocode                                                                                                             
go get -u -v github.com/rogpeppe/godef                                                                                                         
go get -u -v golang.org/x/tools/cmd/godoc                                                                                                      
go get -u -v github.com/zmb3/gogetdoc                                                                                                          
go get -u -v github.com/golang/lint/golint                                                                                                     
go get -u -v github.com/fatih/gomodifytags                                                                                                     
go get -u -v github.com/uudashr/gopkgs/cmd/gopkgs                                                                                              
go get -u -v golang.org/x/tools/cmd/gorename                                                                                                   
go get -u -v sourcegraph.com/sqs/goreturn                                                                                                      
go get -u -v golang.org/x/tools/cmd/goimports                                                                                                  
go get -u -v github.com/cweill/gotests/...                                                                                                     
go get -u -v golang.org/x/tools/cmd/guru                                                                                                       
go get -u -v golang.org/x/lint/golint                                                                                                          
go get -u -v github.com/josharian/impl                                                                                                         
go get -u -v github.com/haya14busa/goplay/cmd/goplay                                                                                           
go get -u -v github.com/uudashr/gopkgs/cmd/gopkgs                                                                                              
go get -u -v github.com/fatih/gomodifytags                                                                                                     
go get -u -v github.com/davidrjenni/reftools/cmd/fillstruct                                                                                    
go get -u -v github.com/sourcegraph/go-langserver                                                                                              
go get -u -v github.com/derekparker/delve/cmd/dlv                                                                                              
go get -u -v github.com/golang/mock/gomock                                                                                                     
go get -u -v github.com/golang/mock/mockgen                                                                                                    
export GO111MODULE=auto