Fixes from polymer.dart API review

Fixes from polymer.dart API review

Primary breaking changes are:
 - polymer/platform.dart is removed
 - Rather than platform.flush, use scheduleMicrotask(Observable.dirtyCheck)
 - Rather than endOfMicrotask, use either scheduleMicrotask or Future.value
 - Polymer.job removed, use Timer instead (and upvote bug 14414 for timer.restart if you really want the job semantics)

This change is bringing up some really weird timing issues in the tests- I don't think any are inherent to the change itself but is more indicative of larger issues.

BUG=13666
R=jmesserly@google.com

Committed: https://code.google.com/p/dart/source/detail?r=29311

[Jennifer Messerly, 2013-10-25 19:24:39]

regarding polymer/lib/job.dart -- it's technically public. Consider at least
making "Job" type internal? Or have it implement Timer and add "restart" method?
(my guess is 14414 unlikely to be fixed for a while)

[Jennifer Messerly, 2013-10-25 19:25:14]

regarding tests: are they flaky now? Or do they work with your changes?

code changes LGTM

[blois, 2013-10-25 20:52:11]

On 2013/10/25 19:25:14, John Messerly wrote:
> regarding tests: are they flaky now? Or do they work with your changes?
> 
> code changes LGTM

Made job private and a part of polymer.

Tests appear to be passing reliably, but the timings are a bit inconsistent
(some use timer-0, some use rAF). I am not confident that there is a definitive
'all the changes are done' event, but I'm also not sure that this is a change
from before.

[Jennifer Messerly, 2013-10-25 21:02:21]

https://codereview.chromium.org/41293002/diff/160001/samples/third_party/todo...
File samples/third_party/todomvc/test/listorder_test.dart (right):

https://codereview.chromium.org/41293002/diff/160001/samples/third_party/todo...
samples/third_party/todomvc/test/listorder_test.dart:36: return
window.animationFrame.then((_) {
one idea here is to use mutation observers to watch for expansion.

[blois, 2013-10-26 16:33:37]

Committed patchset #2 manually as r29311 (presubmit successful).