YAML no longer being written with array indentation

It still works, but setting as "major" since this is a pretty common API:

v0.4 output: http://cgit.drupalcode.org/pants/tree/pants.routing.yml?id=b1b27b192e02d...

pants.change:
  path: '/pants/change/{account}'
  defaults:
    _title: 'Change pants'
    _content: 'Drupal\pants\Controller\DefaultController::change'
  requirements:
    _custom_access: 'Drupal\pants\Controller\DefaultController::changeAccess'
pants.settings:
  path: /admin/config/people/pants
  defaults:
    _title: Pants
    _form: Drupal\pants\Form\PantsSettings
  requirements:
    _permission: 'administer pants'

master output:

pants.change:
    path: '/pants/change/{account}'
    defaults: { _title: 'Change pants', _content: '\Drupal\pants\Controller\DefaultController::pants_change' }
    requirements: { _custom_access: '\Drupal\pants\Controller\DefaultController::pants_change_access' }
pants.settings:
    path: /admin/config/people/pants
    defaults: { _title: Pants, _form: \Drupal\pants\Form\PantsSettings }
    requirements: { _permission: 'administer pants' }

v0.4 was correct.