Make the argument to dispatch_event_with_target non-optional.

[Ms2ger]

The name of the method makes it clear it's supposed to be used with a target
override, so we might as well enforce that.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3549

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[Manishearth]

r+

[Ms2ger]

@bors: retry

• servo#3595

[bors-servo]

saw approval from Manishearth
at Ms2ger@bce3b17

[bors-servo]

merging Ms2ger/servo/dispatch_event_with_target = bce3b17 into auto

[bors-servo]

Ms2ger/servo/dispatch_event_with_target = bce3b17 merged ok, testing candidate = 771be4d

[bors-servo]

some tests failed:
failure: http://build.servo.org/builders/linux2/builds/371

[bors-servo]

saw approval from Manishearth
at Ms2ger@bce3b17

[bors-servo]

merging Ms2ger/servo/dispatch_event_with_target = bce3b17 into auto

[bors-servo]

Ms2ger/servo/dispatch_event_with_target = bce3b17 merged ok, testing candidate = 11b2736

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/373
success: http://build.servo.org/builders/linux2/builds/372
success: http://build.servo.org/builders/android/builds/408
success: http://build.servo.org/builders/mac1/builds/589
success: http://build.servo.org/builders/mac2/builds/590

[bors-servo]

fast-forwarding master to auto = 11b2736