Merge pull request dart-lang/yaml#19 from denesalmasi/yaml

The yaml scanner hung in an infinite loop when trying to scan this
string: "{" (a single opening brace).

_fetchMoreTokens kept growing the token list with a STREAM_END token ad
infinitum.
tree: aa756a07b669d48b8da7794b2afcd5da46ac85f5
  1. pkgs/