incorporate efortuna's review comments into ch03

Review URL: https://chromiumcodereview.appspot.com/110853012
Incorporate review comments to ch03 (library tour).

First batch: lrn's review of dart:core, dart:async, dart:convert

Review URL: https://chromiumcodereview.appspot.com/106363006
Dartiverse Search walkthrough (ch05)

Review URL: https://chromiumcodereview.appspot.com/119023002
add IDs to ch05

Review URL: https://chromiumcodereview.appspot.com/119643003
https://book2-dot-dart-lang.appspot.com/docs/dart-up-and-running/contents/ch04.html
(and following pages)

BUG= https://code.google.com/p/dart/issues/detail?id=9987

Review URL: https://chromiumcodereview.appspot.com/101413003
incorporated gilad's review comments into ch02

Review URL: https://chromiumcodereview.appspot.com/107083005
tweaks to ch02 & ch03

Review URL: https://chromiumcodereview.appspot.com/112043004
make Person's fields final, to justify hashCode

Review URL: https://chromiumcodereview.appspot.com/99583013
Update ch01

Review URL: https://chromiumcodereview.appspot.com/120133003