Fix most defaultPrevented test failures (#2172)

[Manishearth]

#2172

All subtests of dom/events/Event-defaultPrevented.html pass now.

CustomEvent has yet to be implemented.(#2173)

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1319

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[Ms2ger]

r+

[bors-servo]

saw approval from Ms2ger
at Manishearth@fa7bc7a

[bors-servo]

merging Manishearth/servo/domtests-createevent = fa7bc7a into auto

[bors-servo]

Manishearth/servo/domtests-createevent = fa7bc7a merged ok, testing candidate = 4331fad

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1249
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1203

[bors-servo]

fast-forwarding master to auto = 4331fad