name and make pair constructor accessible

Author: RohitSaily

sdk/lib/core/map.dart

@@ -478,7 +478,7 @@ abstract interface class Map<K, V> {
 /// ]);
 /// print(map); // {1: A, 2: B, 3: C, 4: D}
 /// ```
-extension type const MapEntry<K, V>._((K, V) asPair) implements Object {
+extension type const MapEntry<K, V>.fromPair((K, V) asPair) implements Object {
   /// The key of the entry.
   ///
   /// ```dart
@@ -498,5 +498,5 @@ extension type const MapEntry<K, V>._((K, V) asPair) implements Object {
   V get value => asPair.$2;
 
   /// Creates an entry with [key] and [value].
-  const MapEntry(K key, V value): this._((key, value));
+  const MapEntry(K key, V value): this.fromPair((key, value));
 }