Add a string_scanner package. This code has been pulled out of the Shelf package. R=kevmoo@google.com Review URL: https://codereview.chromium.org//213833013 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/string_scanner@34665 260f80e4-7a28-3924-810f-c04153c831b5