In keyword table, change symbols to numbers

Staged at:
https://table-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch02.html#keywords

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1148
R=shaza@google.com

Review URL: https://chromiumcodereview.appspot.com/754033003