Issue 27515. Report DUPLICATE_PAR when the same library contains two part directives with the same …

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/error/error.dart';
 import 'package:analyzer/src/error/codes.dart';
@@ skipping 1691 matching lines @@
     verify([source]);
   }
 
   void test_duplicateDefinition_getter() {
     Source source = addSource("bool get a => true;");
     computeLibrarySourceErrors(source);
     assertNoErrors(source);
     verify([source]);
   }
 
+  void test_duplicatePart() {
+    addNamedSource('/part1.dart', 'part of lib;');
+    addNamedSource('/part2.dart', 'part of lib;');
+    Source source = addSource(r'''
+library lib;
+part 'part1.dart';
+part 'part2.dart';
+''');
+    computeLibrarySourceErrors(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
   void test_dynamicIdentifier() {
     Source source = addSource(r'''
 main() {
   var v = dynamic;
 }''');
     computeLibrarySourceErrors(source);
     assertNoErrors(source);
     verify([source]);
   }
 
@@ skipping 4492 matching lines @@
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "a");
   }
 
   CompilationUnit _getResolvedLibraryUnit(Source source) =>
       analysisContext.getResolvedCompilationUnit2(source, source);
 }