Fix escaping in YAML RegExp. This was causing it to try to parse "m4" as a number, which fails. Fixes dart-lang/yaml#9128. BUG= Review URL: https://codereview.chromium.org//12876003 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/yaml@20107 260f80e4-7a28-3924-810f-c04153c831b5