Adding a lazy transformer example.

Adding a lazy transformer example.

BUG=
R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40008

[Shams, 2014-09-08 19:23:48]

shaza@google.com changed reviewers:
+ nweiz@google.com, rnystrom@google.com

[Shams, 2014-09-08 19:23:49]

I've added a simple lazy transformer example. The text will emphasize that this
transformer doesn't really need to be lazy, unless it were converting, say, the
entire library of congress.

[nweiz, 2014-09-08 22:47:46]

It would be nice to have some prose indicating that usually only transformers
that take a long time should be made lazy, but LGTM.

[Shams, 2014-09-09 00:43:17]

Committed patchset #2 (id:20001) manually as 40008 (presubmit successful).