gr-reply-dialog_test.html fails when dom=shadow in Safari due to a race
condition. Polymer lazily performs local DOM mutations in some cases
for performance (in this case, the two calls to MockInteractions.tap()
were the culprit). flush() ensures that asynchronous changes have taken
place.
Bug: Issue 4573
Change-Id: I5a97ddcc7dd17933f763c4ef58235a0fdd8f5269