Remove usage of dart:json.

R=jmesserly@google.com, lrn@google.com, nweiz@google.com, rnystrom@google.com

Review URL: https://codereview.chromium.org//23596007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@26789 260f80e4-7a28-3924-810f-c04153c831b5