Clarify changelog's Dartium/DDC text
Clarify changelog's Dartium/DDC text

Also made a few minor copy edits.

PiperOrigin-RevId: 182225488

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