Mark two polymer tests as failing/flaky.

Mark two polymer tests as failing/flaky.

Committed: https://code.google.com/p/dart/source/detail?r=38282

[floitsch, 2014-07-16 12:54:43]

TBR.

These two tests still act up.

On IE we fall back to the polyfilled mutation observer. Unless that one is using
window.setImmediate we should catch that and instead use window.setImmediate
ourselves.
Not sure if that's the reason for the failure.

The two-way-binding seems to run into timing issues: The test waits with "new
Future()" for the changes to propagate. new Future uses window.setTimeout(0) and
apparently that sometimes isn't enough. Not sure if changing the test is
acceptable or not.

Note: some of these tests are marked as flaky on Dartium too.

[floitsch, 2014-07-16 12:55:10]

Committed patchset #2 manually as r38282 (presubmit successful).

[Siggi Cherem (dart-lang), 2014-07-16 19:52:36]

lgtm