fix(Forms): fix warning caused by directive host property binding
fix(Forms): fix warning caused by directive host property binding

See https://github.com/dart-lang/angular/issues/1603 for a description of the
issue and why this workaround is necessary.

PiperOrigin-RevId: 211008486

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