message: Use type password for token field
diff: <div md-dialog-content>Please enter the course's access key:</div>
  <div>
  <md-input-container class="full-width">
  - <input async mdInput id="accessKey" placeholder="access key" [(ngModel)]="accessKey"
  + <input async mdInput type="password" id="accessKey" placeholder="access key" [(ngModel)]="accessKey"
  [ngModelOptions]="{standalone: true}">
  </md-input-container>
  </div>
  