don't lint unnecessary lib names pre unnamedLibraries

Related to: dart-lang/lints#181

Change-Id: I4a247fa93c6659d5082d2cb6047aedc6a13a4209
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/359321
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

pkg/linter/lib/src/rules/unnecessary_library_name.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 
@@ -65,6 +66,11 @@ class UnnecessaryLibraryName extends LintRule {
   @override
   void registerNodeProcessors(
       NodeLintRegistry registry, LinterContext context) {
+    if (!context.currentUnit.unit.featureSet
+        .isEnabled(Feature.unnamedLibraries)) {
+      return;
+    }
+
     var visitor = _Visitor(this);
     registry.addLibraryDirective(this, visitor);
   }

pkg/linter/test/rules/unnecessary_library_name_test.dart

@@ -25,6 +25,13 @@ library name;
     ]);
   }
 
+  test_namedLibrary_preUnnamedLibraries() async {
+    await assertNoDiagnostics(r'''
+// @dart = 2.14
+library name;
+''');
+  }
+
   test_unnamedLibrary() async {
     await assertNoDiagnostics(r'''
 library;