The vm service should be more careful when printing the type of a field. We need to decide if returning a class for a field is appropriate in the first place.

* thread #­2: tid = 0x2709a0, 0x92acb952 libsystem_kernel.dylib__pthread_kill + 10, stop reason = signal SIGABRT     frame #­0: 0x92acb952 libsystem_kernel.dylib__pthread_kill + 10
libsystem_kernel.dylib__pthread_kill + 10: -> 0x92acb952: jae 0x92acb962 ; \__pthread_kill + 26    0x92acb954: calll 0x92acb959 ; \__pthread_kill + 17    0x92acb959: popl %edx    0x92acb95a: movl 230807239(%edx), %edx (lldb) bt \* thread #­2: tid = 0x2709a0, 0x92acb952 libsystem_kernel.dylib__pthread_kill + 10, stop reason = signal SIGABRT
    frame #­0: 0x92acb952 libsystem_kernel.dylib__pthread_kill + 10     frame #­1: 0x964c8167 libsystem_pthread.dylibpthread_kill + 101
    frame #­2: 0x92d3629c libsystem_c.dylibabort + 155     frame #­3: 0x000551d5 dartdart::DynamicAssertionHelper::Fail(this=<unavailable>, format=<unavailable>) + 917 at assert.cc:40
    frame #­4: 0x0033ed73 dartdart::AbstractType::type_class(this=0xb0182b98) const + 67 at object.cc:12168     frame #­5: 0x002fc631 dartdart::Field::PrintToJSONStream(this=<unavailable>, stream=0xb01824d0, ref=<unavailable>) const + 1441 at object.cc:6391
    frame #­6: 0x002bc40c dartdart::JSONStream::PrintProperty(char const\*, dart::Object const&, bool) [inlined] dart::JSONStream::PrintValue(this=0xb01824d0, o=<unavailable>) + 108 at json_stream.cc:226     frame #­7: 0x002bc3b9 dartdart::JSONStream::PrintProperty(this=0xb01824d0, name=0x006680f3, o=<unavailable>, ref=<unavailable>) + 25 at json_stream.cc:306
    frame #­8: 0x0033ec10 dartdart::Instance::PrintToJSONStream(dart::JSONStream\*, bool) const [inlined] dart::JSONObject::AddProperty(this=0xb01824d0, ref=true, name=0x006680f3) const + 2112 at json_stream.h:143     frame #­9: 0x0033ebf2 dartdart::Instance::PrintToJSONStream(this=0x00000100, stream=0xb01824d0, ref=true) const + 2082 at object.cc:12141
    frame #­10: 0x00404cd5 dartdart::HandleObjects(isolate=<unavailable>, js=<unavailable>) + 885 at service.cc:934     frame #­11: 0x004029ad dartdart::Service::HandleIsolateMessage(isolate=<unavailable>, msg=<unavailable>) + 1613 at service.cc:583
    frame #­12: 0x002b4a4b dartdart::IsolateMessageHandler::HandleMessage(this=<unavailable>, message=0x07d57c40) + 1323 at isolate.cc:151     frame #­13: 0x002bee5b dartdart::MessageHandler::HandleMessages(this=0x00d61e20, allow_normal_messages=<unavailable>, allow_multiple_normal_messages=<unavailable>) + 187 at message_handler.cc:153
    frame #­14: 0x002bf014 dartdart::MessageHandler::HandleOOBMessages(this=0x00d61e20) + 68 at message_handler.cc:187     frame #­15: 0x00160958 dartdart::DRT_StackOverflow(dart::NativeArguments) [inlined] dart::NativeArguments::isolate(arguments=NativeArguments at 0xb0183a60, this=0x0095f548, this=0x0095f548, this=<unavailable>, isolate=0x01069200, isolate=0x01069200) const + 261 at code_generator.cc:1157
    frame #­16: 0x00160853 dart`dart::DRT_StackOverflow(arguments=NativeArguments at 0xb0183b60) + 515 at code_generator.cc:1131
    frame #­17: 0x00c4906c
    frame #­18: 0x16d65817