Add logging system to pub and sprinkle some logging in.

We'll have to tune which things we log over time, but this covers
most IO operations which is where almost all of our problems come
from.

This patch also means that going forward you should no longer call
print() or printError() (which is gone) directly. Instead:

print() -> log.message()
printError() -> log.error()

That ensures all messages will correctly get added to the log's
transcript.

Review URL: https://codereview.chromium.org//11437019

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@15827 260f80e4-7a28-3924-810f-c04153c831b5