rehabilitate the clock sample a bit

Some tweaks to the clock sample:
- change parts into libraries
- move most of the sample code into `lib/` (from `web/`)
- made some things private; changed others to use the function shorthand
- update the pubspec dependencies

@sigmundch

Also - somewhat related - I remember you saying that the samples would be tested via dart's buildbots. Do you know, has there been any discussion about also testing them with drone.io/travis/other bots? These ones are more familiar to the larger dart community and ~easier for them to work with.