Make CanonicalizedMap.cast use the public key and value types

pkgs/collection/CHANGELOG.md

@@ -1,5 +1,6 @@
 ## 1.20.0-wip
 
+- `CanonicalizedMap.cast` returns a view of the public keys and values.
 - Adds `separated` and `separatedList` extension methods to `Iterable`.
 - Adds `separate` extension method to `List`
 - Add `IterableMapEntryExtension` for working on `Map` as a list of pairs, using

pkgs/collection/lib/src/canonicalized_map.dart

@@ -106,7 +106,11 @@ class CanonicalizedMap<C, K, V> implements Map<K, V> {
       );
 
   @override
-  Map<K2, V2> cast<K2, V2>() => _base.cast<K2, V2>();
+  Map<K2, V2> cast<K2, V2>() {
+    // `_base` stores canonical keys and `MapEntry`s, not the public `K`/`V`
+    // view. Casting that storage makes lookups fail with a cast error.
+    return Map.castFrom<K, V, K2, V2>(this);
+  }
 
   @override
   void clear() {

pkgs/collection/test/canonicalized_map_test.dart

@@ -159,6 +159,14 @@ void main() {
     test('cast returns a new map instance', () {
       expect(map.cast<Pattern, Pattern>(), isNot(same(map)));
     });
+
+    test('cast preserves lookups on the public key and value types', () {
+      map.addAll({'01': 'value 01', '2': 'value 2'});
+      final casted = map.cast<String, String>();
+      expect(casted['01'], 'value 01');
+      expect(casted['1'], 'value 01');
+      expect(casted.length, 2);
+    });
   });
 
   group('CanonicalizedMap builds an informative string representation', () {