Don't flag Never as not available when the non-nullable feature is enabled for the library.

R=brianwilkerson@google.com

Bug: https://github.com/dart-lang/sdk/issues/39597
Change-Id: I5865f467a3b5097fb863d172321f60cbe1f5fd44
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132023
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/src/hint/sdk_constraint_verifier.dart

@@ -101,9 +101,7 @@ class SdkConstraintVerifier extends RecursiveAstVisitor<void> {
 
   /// Return `true` if references to the non-nullable features need to be
   /// checked.
-  // TODO(brianwilkerson) Implement this as a version check when a version has
-  //  been selected.
-  bool get checkNnbd => true;
+  bool get checkNnbd => !_containingLibrary.isNonNullableByDefault;
 
   /// Return `true` if references to set literals need to be checked.
   bool get checkSetLiterals =>

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

@@ -12,6 +12,7 @@ import 'sdk_constraint_verifier_support.dart';
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(SdkVersionNeverTest);
+    defineReflectiveTests(SdkVersionNeverWithNnbdTest);
   });
 }
 
@@ -19,32 +20,36 @@ main() {
 class SdkVersionNeverTest extends SdkConstraintVerifierTest {
   @override
   AnalysisOptionsImpl get analysisOptions => AnalysisOptionsImpl()
-    ..contextFeatures = FeatureSet.forTesting(
-        sdkVersion: '2.3.0', additionalFeatures: [Feature.non_nullable]);
+    ..contextFeatures = FeatureSet.forTesting(sdkVersion: '2.7.0');
 
-  @failingTest
-  test_equals() async {
-    // This test cannot pass because there is no version number that is equal to
-    // when non-nullable was enabled.
-    await verifyVersion('2.1.0', '''
-Never sink;
-''');
-  }
-
-  @failingTest
-  test_greaterThan() async {
-    // This test cannot pass because there is no version number that is equal to
-    // when non-nullable was enabled.
-    await verifyVersion('2.1.0', '''
-Never sink;
-''');
-  }
-
-  test_lessThan() async {
-    await verifyVersion('2.3.0', '''
-Never sink = (throw 42);
+  test_languageVersionBeforeNullSafety() async {
+    await verifyVersion('2.7.0', r'''
+Never foo;
 ''', expectedErrors: [
       error(HintCode.SDK_VERSION_NEVER, 0, 5),
     ]);
   }
 }
+
+@reflectiveTest
+class SdkVersionNeverWithNnbdTest extends SdkConstraintVerifierTest {
+  @override
+  AnalysisOptionsImpl get analysisOptions => AnalysisOptionsImpl()
+    ..contextFeatures =
+        FeatureSet.forTesting(additionalFeatures: [Feature.non_nullable]);
+
+  test_experimentEnabled() async {
+    await verifyVersion('2.7.0', r'''
+Never foo = (throw 42);
+''');
+  }
+
+  test_experimentEnabled_libraryOptedOut() async {
+    await verifyVersion('2.7.0', r'''
+// @dart = 2.7
+Never foo = (throw 42);
+''', expectedErrors: [
+      error(HintCode.SDK_VERSION_NEVER, 15, 5),
+    ]);
+  }
+}