linter: move cast_nullable_to_non_nullable tests

Change-Id: I5bd5482108b9c0bb29936819ef4151d6b947a4b5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/366080
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/linter/test/rules/cast_nullable_to_non_nullable_test.dart

@@ -17,7 +17,23 @@ class CastNullableToNonNullableTest extends LintRuleTest {
   @override
   String get lintRule => 'cast_nullable_to_non_nullable';
 
-  test_castNullable() async {
+  test_castDynamic_toNonNullable() async {
+    await assertNoDiagnostics(r'''
+void f(dynamic a) {
+  a as String;
+}
+''');
+  }
+
+  test_castDynamic_toNullable() async {
+    await assertNoDiagnostics(r'''
+void f(dynamic a) {
+  a as String?;
+}
+''');
+  }
+
+  test_castNullable_toNonNullable() async {
     await assertDiagnostics(r'''
 String? s;
 var a = s as String;
@@ -26,6 +42,17 @@ var a = s as String;
     ]);
   }
 
+  test_castNullable_toNullable() async {
+    await assertNoDiagnostics(r'''
+class A {}
+class B extends A {}
+
+void f(A? a) {
+  a as B?;
+}
+''');
+  }
+
   test_castNullable_unresolvedType() async {
     await assertDiagnostics(r'''
 Undefined? s;