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.

[email protected]

Change-Id: I0e609656fccd563ecf71d4be5ecadafc1a6da891
CoreLibraryReviewExempt: Doc changes only.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310774
Commit-Queue: Nate Bosch <[email protected]>
Reviewed-by: Lasse Nielsen <[email protected]>

Author: natebosch

sdk/lib/collection/hash_map.dart

@@ -38,6 +38,10 @@ typedef _Hasher<K> = int Function(K object);
 /// called during a [forEach] or [putIfAbsent] call,
 /// or while iterating the map ([keys], [values] or [entries]).
 ///
+/// Do not modify keys in any way which changes their equality (and thus their
+/// hash code) while they are in the map. If a map key's [Object.hashCode]
+/// changes, it may cause future lookups for that key to fail.
+///
 /// Example:
 /// ```dart
 /// final Map<int, String> planets = HashMap(); // Is a HashMap