Implement HasToJson for change objects.

Plus tests for these objects.

R=paulberry@google.com
BUG=

Review URL: https://codereview.chromium.org//407993002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38448 260f80e4-7a28-3924-810f-c04153c831b5