[element model] fix unsafe ConstructorMember cast

See: b/374689139.

https://dart-review.googlesource.com/c/sdk/+/390941 is blocking an SDK roll.

Root cause:


```
Action threw an exception: type 'ConstructorMember' is not a subtype of type 'ConstructorFragment' in type cast
#0      InterfaceTypeImpl.constructors2.<anonymous closure> (package:analyzer/src/dart/element/type.dart:569)
#1      MappedListIterable.elementAt (dart:_internal/iterable.dart:435)
#2      ListIterator.moveNext (dart:_internal/iterable.dart:364)
#3      new _GrowableList._ofEfficientLengthIterable (dart:core-patch/growable_array.dart:189)
#4      new _GrowableList.of (dart:core-patch/growable_array.dart:150)
#5      new List.of (dart:core-patch/array_patch.dart:39)
#6      ListIterable.toList (dart:_internal/iterable.dart:224)
#7      InterfaceTypeImpl.constructors2 (package:analyzer/src/dart/element/type.dart:570)
#8      _Visitor._hasConstConstructorInvocation (package:linter/src/rules/prefer_const_constructors_in_immutables.dart:110)
#9      _Visitor.visitConstructorDeclaration (package:linter/src/rules/prefer_const_constructors_in_immutables.dart:58)
```


To verify the fix locally:

```
  solo_test_X() async {
    await assertNoErrorsInCode(r'''
class A<T> {}
''');

    var A = findElement.class_('A').instantiate(
      typeArguments: [intType],
      nullabilitySuffix: NullabilitySuffix.none,
    );
    A.constructors2;
  }
```


Bug: b/374689139
Change-Id: I70034d938d840dc0c3939db27e7116164e4617e9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391483
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/analyzer/lib/src/dart/element/type.dart

@@ -554,7 +554,13 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
 
   @override
   List<ConstructorElement2> get constructors2 => constructors
-      .map((fragment) => (fragment as ConstructorFragment).element)
+      .map((fragment) => switch (fragment) {
+            ConstructorFragment(:var element) => element,
+            ConstructorMember() => fragment,
+            _ => throw StateError(
+                'unexpected fragment type: ${fragment.runtimeType}',
+              )
+          })
       .toList();
 
   @override

pkg/linter/test/rules/prefer_const_constructors_in_immutables_test.dart

@@ -405,6 +405,21 @@ macro class M {
     ]);
   }
 
+  test_parameterizedType() async {
+    // Verify we aren't doing an unsafe cast to a `ConstructorFragment` in type.dart.
+    // b/374689139
+    await assertNoDiagnostics(r'''
+import 'package:meta/meta.dart';
+
+class A<T> {}
+
+@immutable
+class C<U> extends A<U> {
+  C();
+}
+''');
+  }
+
   test_returnOfInvalidType() async {
     await assertDiagnostics(r'''
 import 'package:meta/meta.dart';