chore(travis): use latest Dart SDK (#2320)
chore(travis): use latest Dart SDK (#2320)

Dartdoc has been fixed so we no longer need to be pinned to SDK v1.18.1.
angular/angular-cn,GuzmanPI/angular-es,rsreimer/angular.io,angular/angular-ko,juleskremer/angular.io,onyoon7/angular-ko,kwalrath/angular.io,onyoon7/angular-ko,angular/angular-ko,chalin/angular.io,angular/angular-ko,angular/angular-es,angular/angular.io,angular/angular-es,realappie/angular.io,Urigo/angular.io,realappie/angular.io,IdeaBlade/angular.io,chalin/angular.io,realappie/angular.io,realappie/angular.io,onyoon7/angular-ko,Urigo/angular.io,kwalrath/angular.io,angular/angular-cn,juleskremer/angular.io,Urigo/angular.io,IdeaBlade/angular.io,juleskremer/angular.io,kwalrath/angular.io,angular/angular-cn,angular/angular.io,rsreimer/angular.io,angular/angular-es,GuzmanPI/angular-es,chalin/angular.io,angular/angular-es,chalin/angular.io,angular/angular.io,angular/angular.io,realappie/angular.io,rsreimer/angular.io,rsreimer/angular.io,kwalrath/angular.io,GuzmanPI/angular-es,chalin/angular.io,kwalrath/angular.io,angular/angular.io,angular/angular-ko,rsreimer/angular.io,IdeaBlade/angular.io,Urigo/angular.io,onyoon7/angular-ko,angular/angular-ko,GuzmanPI/angular-es,juleskremer/angular.io,IdeaBlade/angular.io,GuzmanPI/angular-es,IdeaBlade/angular.io,Urigo/angular.io,angular/angular-cn,juleskremer/angular.io,angular/angular-cn,angular/angular-es,onyoon7/angular-ko