Rewrite the pkg/yaml parser.

This increases the speed by about 10x. It also adds a number of new featurse.

BUG=21317, 20859
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//689513002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41649 260f80e4-7a28-3924-810f-c04153c831b5