`Alt + D` on an application that was run *without* `--debug` breaks

[grouma]

Unhandled exception:
NoSuchMethodError: The getter 'hostname' was called on null.
Receiver: null
Tried calling: hostname
#0      Object.noSuchMethod (dart:core/runtime/lib/object_patch.dart:50:5)
#1      DevHandler._handleConnection.<anonymous closure> (package:webdev/src/serve/handlers/dev_handler.dart:117:51)
<asynchronous suspension>
#2      _rootRunUnary (dart:async/zone.dart:1136:13)
#3      _RootZone.runUnaryGuarded (dart:async/zone.dart:1317:7)
#4      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#5      _DelayedData.perform (dart:async/stream_impl.dart:591:14)
#6      _StreamImplEvents.handleNext (dart:async/stream_impl.dart:707:11)
#7      _PendingEvents.schedule.<anonymous closure> (dart:async/stream_impl.dart:667:7)
#8      _rootRun (dart:async/zone.dart:1120:38)
#9      _CustomZone.run (dart:async/zone.dart:1021:19)
#10     _CustomZone.runGuarded (dart:async/zone.dart:923:7)
#11     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:963:23)
#12     _rootRun (dart:async/zone.dart:1124:13)
#13     _CustomZone.run (dart:async/zone.dart:1021:19)
#14     _CustomZone.runGuarded (dart:async/zone.dart:923:7)
#15     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:963:23)
#16     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#17     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
#18     _runPendingImmediateCallback (dart:isolate/runtime/lib/isolate_patch.dart:115:13)
#19     _RawReceivePortImpl._handleMessage (dart:isolate/runtime/lib/isolate_patch.dart:172:5)

Problematic PR: https://github.com/dart-lang/webdev/pull/222/files