Add an UNUSED_SHOWN_NAMES hint to the analyzer

pkg/analyzer/test/generated/non_error_resolver_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library analyzer.test.generated.non_error_resolver_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 149 matching lines @@
 library lib1;
 class N {}
 class N1 {}''');
     addNamedSource(
         "/lib2.dart",
         r'''
 library lib2;
 class N {}
 class N2 {}''');
     computeLibrarySourceErrors(source);
-    assertNoErrors(source);
+    assertErrors(source, [HintCode.UNUSED_SHOWN_NAME]);
   }
 
   void test_annotated_partOfDeclaration() {
     Source source = addSource('library L; part "part.dart";');
     addNamedSource('/part.dart', '@deprecated part of L;');
     computeLibrarySourceErrors(source);
     assertNoErrors(source);
     verify([source]);
   }
 
@@ skipping 5907 matching lines @@
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "a");
   }
 
   CompilationUnit _getResolvedLibraryUnit(Source source) =>
       analysisContext.getResolvedCompilationUnit2(source, source);
 }