Crash w/ coverage on json_serializable

[kevmoo]

This is w/ json_serial @ https://github.com/dart-lang/json_serializable/commit/13547e6dee95572541a09ed77e495c06adcbbb08

json_serializable/json_serializable/ (master $)> pub run test -p vm  --coverage tmp_cvg
Warning: You should set `SILENT_OBSERVATORY` to true when debugging the VM as it will output the observatory URL by default.
This breaks the various reporter contracts.
To set the value define `DART_VM_OPTIONS=-DSILENT_OBSERVATORY=true`.

00:00 +0: test/ensure_build_test.dart: (suite)
  Skip: Should only be run during presubmit
00:00 +0 ~1: loading test/config_test.dart                                                                                                                                                                                                                                     The getter 'fragment' was called on null.
Receiver: null
Tried calling: fragment
dart:core                                         Object.noSuchMethod
package:test_core/src/runner/coverage.dart 21:71  gatherCoverage
package:test_core/src/runner/engine.dart 287:40   Engine.run.<fn>.<fn>.<fn>
package:pool/pool.dart 127:28                     Pool.withResource
package:test_core/src/runner/engine.dart 283:24   Engine.run.<fn>.<fn>
package:test_core/src/runner/engine.dart 290:8    Engine.run.<fn>

This is an unexpected error. Please file an issue at http://github.com/dart-lang/test
with the stack trace and instructions for reproducing the error.
Unhandled exception:
NoSuchMethodError: The getter 'fragment' was called on null.
Receiver: null
Tried calling: fragment
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      gatherCoverage (package:test_core/src/runner/coverage.dart:21:71)
<asynchronous suspension>
#2      Engine.run.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:test_core/src/runner/engine.dart:287:40)
<asynchronous suspension>
#3      Pool.withResource (package:pool/pool.dart:127:28)
<asynchronous suspension>
#4      Engine.run.<anonymous closure>.<anonymous closure> (package:test_core/src/runner/engine.dart:283:24)
<asynchronous suspension>
#5      Engine.run.<anonymous closure> (package:test_core/src/runner/engine.dart:290:8)
#6      _rootRunUnary (dart:async/zone.dart:1132:38)
#7      _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#8      _CustomZone.runUnaryGuarded (dart:async/zone.dart:931:7)
#9      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#10     _DelayedData.perform (dart:async/stream_impl.dart:591:14)
#11     _StreamImplEvents.handleNext (dart:async/stream_impl.dart:707:11)
#12     _PendingEvents.schedule.<anonymous closure> (dart:async/stream_impl.dart:667:7)
#13     _rootRun (dart:async/zone.dart:1124:13)
#14     _CustomZone.run (dart:async/zone.dart:1021:19)
#15     _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:947:23)
#16     _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#17     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
#18     _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:116:13)
#19     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:173:5)