After careful consideration by the migration system maintainers, and consultation with xjm (D8 Release Manager), we've decided that migration of legacy views will not be supported in Drupal 8 core for the initial stable release of the core Migrate suite. heddn's comment lays out many of the specific challenges this support would face. Simply put, a fully complete automated migration path for views is not possible, and implementing what is possible would take too much time and effort away from more tractable migration work only to arrive at a situation where we end up saying "views migration is supported, except for views defined in code, and views which use these plugins, and views based on that table, ...". A partial solution is not suitable for core.
That being said, dawehner has started a migrate_views project in contrib - we would encourage those who are motivated to tackle this difficult problem to help out there. A contributed module can more flexibly attack the issue incrementally, for example starting with support for simple node views and enhancing the support from there over time.
Thanks for all your input.