[vm] Const map, remove invalid assert
[vm] Const map, remove invalid assert

Isolates are allowed to race each other to initialize _indexNullable,
the last isolate wins. See
https://dart-review.googlesource.com/c/sdk/+/210726 and
https://dart-review.googlesource.com/c/sdk/+/203765.

These asserts were wrong, because another isolate could have initialized
_indexNullable in between places calling _createIndex and _createIndex
itself.

Closes: https://github.com/dart-lang/sdk/issues/47118

Change-Id: I5d55b9715ef622737a6f70c0b57590076ece3225
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212586
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Commit-Queue: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>

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