feat(Build): Add is_optional: true to build.yaml.
feat(Build): Add is_optional: true to build.yaml.

Closes #1002

PiperOrigin-RevId: 187903032

chalin/angular2,chalin/angular2,matanlurey/angular,matanlurey/angular,matanlurey/angular,kevmoo/angular,angulardart/angular,kevmoo/angular,chalin/angular2,kevmoo/angular,angulardart/angular,angulardart/angular