[vm] unmodifiable typed data MSAN error

[dcharkes]

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There are new test failures on [vm] Record field access operations...[dart2wasm] Trivial test for subtype of Object.

The tests

vm/dart_2/isolates/thread_pool_test Crash (expected Pass)

are failing on configurations

dartk-msan-linux-release-x64
dartkp-msan-linux-release-x64

https://dart-ci.appspot.com/log/vm-kernel-msan-linux-release-x64/dartk-msan-linux-release-x64/1856/vm/dart_2/isolates/thread_pool_test

MemorySanitizer: use-of-uninitialized-value

https://github.com/dart-lang/sdk/blob/e4ae0cf2ce0755b85e9bffdfa50f5042e4edcd99/runtime/bin/ffi_test/ffi_test_functions_vmspecific.cc#L899

The code there hasn't changed in a while, so it must be one of the VM commits in the range. There are a bunch of VM commits in the range, will bisect locally.

No repro: 912577baecccce11cbbdfa56ba5bf2a909e3fc6e Repro: 3744763f3a57073bcdbac956f2794feaf0ac050e No repro: 5389fd41ed682cf9a21cebf6bacfa554741f413e No repro: 8f70d95df0accca98b5f06db17009cdefbb325c3 Repro: 8c577c420c43f5fa4af3f077d8715b64d4dfb0c5 <- culprit

[dcharkes]

cc @athomas @whesse I'd really like an auto-bisect button in the Results Feed for bisecting a range of 18 commits w.r.t. a single test that changed status.

[dcharkes]

@rmacnak-google, 8c577c420c43f5fa4af3f077d8715b64d4dfb0c5 causes an MSAN error, PTAL.