Update Timer API to take the callback as the last parameter.

Update Timer API to take the callback as the last parameter.

This makes the code easier to read when the callback actually does
work:

  new Timer(1000, (t) {
    // do
    // lots
    // of
    // stuff
  });

Also, it is more consistent with the rest of the APIs.

R=sgjesse@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=4945

[Søren Gjesse, 2012-03-05 11:32:13]

lgtm