Issue 40764. Use Never instead of Null when converting super-bounded to regular-bounded.

Bug: https://github.com/dart-lang/sdk/issues/40764
Change-Id: I58f0e9860dd000aa2d8e416c5fce7e8964db0ef8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/137280
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

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

@@ -82,7 +82,7 @@ class DynamicTypeImpl extends TypeImpl {
   DartType replaceTopAndBottom(TypeProvider typeProvider,
       {bool isCovariant = true}) {
     if (isCovariant) {
-      return typeProvider.nullType;
+      return NeverTypeImpl.instance;
     } else {
       return this;
     }

pkg/analyzer/test/src/diagnostics/type_argument_not_matching_bounds_test.dart

@@ -450,6 +450,14 @@ class B extends A {}
 main() {
   foo<B, A>();
 }
+''');
+  }
+
+  test_superBounded() async {
+    await assertNoErrorsInCode(r'''
+class A<X extends A<X>> {}
+
+A get foo => throw 0;
 ''');
   }
 }

tests/language/instantiate_to_bound/README.md

@@ -0,0 +1,12 @@
+# Feature tests for instantiation to bounds, and super-bounded types
+
+This directory was created in order to hold tests pertaining to the
+Dart feature _instantiate to bound_, which provides inference of
+default values for omitted type arguments. In order to handle
+F-bounded type parameters without introducing infinite types, this
+feature relies on another feature, _super-bounded types_, which is
+therefore also in focus for tests in this directory. For more details,
+please check the feature specifications on
+[super-bounded types](https://github.com/dart-lang/sdk/blob/master/docs/language/informal/super-bounded-types.md)
+and on
+[instantiate to bound](https://github.com/dart-lang/sdk/blob/master/docs/language/informal/instantiate-to-bound.md).

tests/language/instantiate_to_bound/issue40764_test.dart

@@ -0,0 +1,5 @@
+class A<X extends A<X>> {}
+
+A get g => throw 0;
+
+main() {}