VM AOT crash in collection tests with NNBD changes

[leafpetersen]

This CL makes some changes to the core libraries to eliminate casts in the NNBD splay tree code. The AOT compiled code for corelib/collection_length_test seems to be crashing on the nnbd try bot. See log here

[alexmarkov]

https://dart-review.googlesource.com/c/sdk/+/143310 should fix the tests failing on vm-kernel-precomp-nnbd-linux-release-x64-try bot.