flutter: In function canCallDeferredLibrary
[ERROR:flutter/runtime/dart_isolate.cc(1051)] Platform Configuration was null. Deferred library load requestfor loading unit id 2 was not sent.
[ERROR:flutter/runtime/dart_isolate_unittests.cc(495)] Hello from Dart!
../../third_party/dart/runtime/vm/dart_api_impl.cc: 1542: error: Isolate canCallDeferredLibrary is already scheduled on mutator thread 0x7f96d4018e00, failed to schedule from os thread 0x10fab5dc0

version=2.12.0-150.0.dev (dev) (Mon Dec 14 12:05:40 2020 -0800) on "macos_x64"
pid=24557, thread=775, isolate_group=(nil)(0x0), isolate=(nil)(0x0)
isolate_instructions=0, vm_instructions=10be02000
  pc 0x0000000108943c15 fp 0x00007ffee757a0f0 dart::Profiler::DumpStackTrace(void*)+0x85
  pc 0x0000000108865612 fp 0x00007ffee757a1d0 dart::Assert::Fail(char const*, ...)+0x82
  pc 0x0000000108e71330 fp 0x00007ffee757a210 Dart_ThreadDisableProfiling+0x0
  pc 0x00000001086d4edf fp 0x00007ffee757a230 tonic::DartState::Scope::Scope(tonic::DartState*)+0x1f
  pc 0x00000001086a5a9a fp 0x00007ffee757a2f0 flutter::DartIsolate::LoadLoadingUnit(long, std::__1::unique_ptr<fml::Mapping const, std::__1::default_delete<fml::Mapping const> >, std::__1::unique_ptr<fml::Mapping const, std::__1::default_delete<fml::Mapping const> >)+0x3a
  pc 0x000000010868f067 fp 0x00007ffee757b0a0 flutter::testing::DartIsolateTest_ValidLoadingUnitSucceeds_Test::TestBody()+0x9b7
  pc 0x0000000108ecb560 fp 0x00007ffee757b0e0 testing::Test::Run()+0xe0
  pc 0x0000000108ecc2b0 fp 0x00007ffee757b140 testing::TestInfo::Run()+0x110
  pc 0x0000000108eccd07 fp 0x00007ffee757b190 testing::TestSuite::Run()+0x127
  pc 0x0000000108eda177 fp 0x00007ffee757b250 testing::internal::UnitTestImpl::RunAllTests()+0x3b7
  pc 0x0000000108ed9d0f fp 0x00007ffee757b2a0 testing::UnitTest::Run()+0x7f
  pc 0x00000001086c99ff fp 0x00007ffee757b400 main+0x12f
  pc 0x00007fff73ad2cc9 fp 0x00007ffee757b418 start+0x1
-- End of DumpStackTrace
Traceback (most recent call last):
  File "/opt/s/w/ir/cache/builder/src/flutter/testing/run_tests.py", line 506, in <module>
    sys.exit(main())
  File "/opt/s/w/ir/cache/builder/src/flutter/testing/run_tests.py", line 476, in main
    RunCCTests(build_dir, engine_filter)
  File "/opt/s/w/ir/cache/builder/src/flutter/testing/run_tests.py", line 136, in RunCCTests
    RunEngineExecutable(build_dir, 'runtime_unittests', filter, shuffle_flags)
  File "/opt/s/w/ir/cache/builder/src/flutter/testing/run_tests.py", line 94, in RunEngineExecutable
    RunCmd(test_command, cwd=cwd)
  File "/opt/s/w/ir/cache/builder/src/flutter/testing/run_tests.py", line 47, in RunCmd
    raise Exception('Command "%s" exited with code %d' % (command_string, process.returncode))
Exception: Command "/opt/s/w/ir/cache/builder/src/out/host_profile/runtime_unittests --gtest_shuffle --gtest_repeat=2" exited with code -6```

Not sure what PR has caused it, but I noticed it on a Dart SDK roll - https://dart.googlesource.com/sdk.git/+log/68d1c7504f7d..f166571c7bc4