Update clock dart packaged app example for a modern dart.

This change allows the clock to run in Dartium m31, or in Chrome,
without changes.

Changes:
 dart2js no longer accepts --disallow-unsafe-eval
 window.requestLayoutFrame is now scheudleMicrotask from dart:async.
 Date is now DateTime
 List<E>.filter is now List<E>.removeWhere
 String.charCodeAt is now String.codeUnitAt
 js/interop stuff no longer required

Bugfix: Fix numbers not showing up at all when first opened, e.g., at
exactly 4pm and now.minute == 0.

Review URL: https://codereview.appspot.com/56440043