runtime/vm/object.cc: 3994: error: expected: direct_implementors.At(i) != implementor.raw()

[alexmarkov]

After cleaning up checks during class finalization in 569ee07, co19_2/Language/Classes/definition_t23 test starts crashing in debug mode. This test was previously failing with an unexpected compile-time error.

FAILED: dartk-vm debug_x64 co19_2/Language/Classes/definition_t23
Expected: Pass
Actual: Crash

--- Command "vm" (took 02.000577s):
DART_CONFIGURATION=DebugX64 out/DebugX64/dart --no-background-compilation --ignore-unrecognized-flags --packages=/b/s/w/ir/.packages /b/s/w/ir/tests/co19_2/src/Language/Classes/definition_t23.dart

exit code:
-6

stderr:
../../runtime/vm/object.cc: 3994: error: expected: direct_implementors.At(i) != implementor.raw()
thread=13176, isolate=definition_t23.dart:main()(0x5625eb837200)
  [0x00005625e8bd859c] dart::Profiler::DumpStackTrace(void*)
  [0x00005625e8bd859c] dart::Profiler::DumpStackTrace(void*)
  [0x00005625e8e943b2] dart::Assert::Fail(char const*, ...)
  [0x00005625e8b30acb] dart::Class::AddDirectImplementor(dart::Class const&) const
  [0x00005625e8a26fdf] dart::ClassFinalizer::FinalizeTypesInClass(dart::Class const&)
  [0x00005625e8a2665f] dart::ClassFinalizer::ProcessPendingClasses()
  [0x00005625e8aeb53a] dart::kernel::KernelLoader::LoadProgram(bool)
  [0x00005625e8aeaf30] dart::kernel::KernelLoader::LoadEntireProgram(dart::kernel::Program*, bool)
  [0x00005625e8e8af0b] Dart_LoadScriptFromKernel
  [0x00005625e8812600] Unknown symbol
  [0x00005625e88110e1] Unknown symbol
  [0x00005625e88106e1] dart::bin::RunMainIsolate(char const*, dart::bin::CommandLineOptions*)
  [0x00005625e8811917] dart::bin::main(int, char**)
  [0x00005625e8812319] Unknown symbol
-- End of DumpStackTrace

--- Re-run this test:
python tools/test.py -n dartk-linux-debug-x64 co19_2/Language/Classes/definition_t23