chore(travis): anchor Dart SDK version to 1.18.1 (#2204)
chore(travis): anchor Dart SDK version to 1.18.1 (#2204)

Because dartdoc from 1.19.0 has issues -
https://github.com/dart-lang/dartdoc/issues/1233.
NoahLuftYang/angular-ko,gyujincho/angular-ko,angular/angular-ko,kwalrath/angular.io,IdeaBlade/angular.io,kwalrath/angular.io,realappie/angular.io,angular/angular-cn,slm-sum/angular-ja,angular/angular-cn,SaloGala/angular-es,onyoon7/angular-ko,Urigo/angular.io,GuzmanPI/angular-es,rsreimer/angular.io,slm-sum/angular-ja,NoahLuftYang/angular-ko,Urigo/angular.io,rsreimer/angular.io,angular/angular-es,rsreimer/angular.io,kwalrath/angular.io,naomiblack/angular.io,realappie/angular.io,angular/angular.io,angular/angular-es,Urigo/angular.io,SaloGala/angular-es,onyoon7/angular-ko,juleskremer/angular.io,juleskremer/angular.io,angular/angular-cn,NoahLuftYang/angular-ko,rsreimer/angular.io,chalin/angular.io,juleskremer/angular.io,kwalrath/angular.io,onyoon7/angular-ko,IdeaBlade/angular.io,naomiblack/angular.io,IdeaBlade/angular.io,angular/angular-cn,realappie/angular.io,chalin/angular.io,kwalrath/angular.io,SaloGala/angular-es,chalin/angular.io,IdeaBlade/angular.io,gyujincho/angular-ko,angular/angular.io,IdeaBlade/angular.io,angular/angular-es,naomiblack/angular.io,realappie/angular.io,angular/angular-es,NoahLuftYang/angular-ko,onyoon7/angular-ko,onyoon7/angular-ko,gyujincho/angular-ko,slm-sum/angular-ja,GuzmanPI/angular-es,angular/angular-cn,angular/angular-es,naomiblack/angular.io,NoahLuftYang/angular-ko,juleskremer/angular.io,gyujincho/angular-ko,angular/angular.io,GuzmanPI/angular-es,GuzmanPI/angular-es,angular/angular-ko,gyujincho/angular-ko,angular/angular-ko,realappie/angular.io,SaloGala/angular-es,Urigo/angular.io,angular/angular-ko,Urigo/angular.io,rsreimer/angular.io,angular/angular.io,chalin/angular.io,chalin/angular.io,GuzmanPI/angular-es,naomiblack/angular.io,slm-sum/angular-ja,angular/angular-ko,juleskremer/angular.io,angular/angular.io,SaloGala/angular-es,slm-sum/angular-ja