Add a notice about HashMap keys changing hashCode

Closes #33384

Similar to a notice in the doc for `HashSet` where it is also unsafe to
change `hashCode` while an object is in the collection.

R=lrn@google.com

Change-Id: I0e609656fccd563ecf71d4be5ecadafc1a6da891
CoreLibraryReviewExempt: Doc changes only.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310774
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

https://dart.googlesource.com/sdk/+/34d9241a7194ee30e751f5fb09a159fd99873d39
1 file changed
tree: 132fc49cda8cfdf676b8f82ad21681a36665cfe6
  1. commits.json