[collection] explicitly make BoolList abstract interface

It couldn't be extended in practice since it doesn't have a
public constructor

Also made the only implementations final

Author: kevmoo

pkgs/collection/CHANGELOG.md

@@ -3,6 +3,7 @@
 
 - Add `IterableMapEntryExtension` for working on `Map` as a list of pairs, using
   `Map.entries`.
+- Explicitly mark `BoolList` as `abstract interface`
 - Address diagnostics from `strict_top_level_inference`.
 
 ## 1.19.1

pkgs/collection/lib/src/boollist.dart

@@ -10,7 +10,7 @@ import 'unmodifiable_wrappers.dart' show NonGrowableListMixin;
 /// A space-efficient list of boolean values.
 ///
 /// Uses list of integers as internal storage to reduce memory usage.
-abstract /*mixin*/ class BoolList with ListMixin<bool> {
+abstract interface class BoolList with ListMixin<bool> {
   static const int _entryShift = 5;
 
   static const int _bitsPerEntry = 32;
@@ -180,7 +180,7 @@ abstract /*mixin*/ class BoolList with ListMixin<bool> {
   }
 }
 
-class _GrowableBoolList extends BoolList {
+final class _GrowableBoolList extends BoolList {
   static const int _growthFactor = 2;
 
   _GrowableBoolList._withCapacity(int length, int capacity)
@@ -228,7 +228,8 @@ class _GrowableBoolList extends BoolList {
   }
 }
 
-class _NonGrowableBoolList extends BoolList with NonGrowableListMixin<bool> {
+final class _NonGrowableBoolList extends BoolList
+    with NonGrowableListMixin<bool> {
   _NonGrowableBoolList._withCapacity(int length, int capacity)
       : super._(
           Uint32List(BoolList._lengthInWords(capacity)),