This also removes one bit of magic to make it more obvious what on is
going on during a sync, which should hopefully help.

Components have to decide if they support being stateful or not. If
they do, then they must implement syncFields() and have mutable
fields; if they don't, then they must have final fields. This isn't
particularly enforced, though.

This also renames _willSync() to _retainStatefulNodeIfPossible(), for
clarity, and fixes some minor style issues and one typo that was
breaking the drawer.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1174023003