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

pkg/analyzer/test/generated/compile_time_error_code_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.compile_time_error_code_test;
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/parser.dart' show ParserErrorCode;
@@ skipping 1941 matching lines @@
     Source source = addSource(r'''
 f({a, b}) {}
 main() {
   f(a: 1, a: 2);
 }''');
     computeLibrarySourceErrors(source);
     assertErrors(source, [CompileTimeErrorCode.DUPLICATE_NAMED_ARGUMENT]);
     verify([source]);
   }
 
+  void test_duplicatePart() {
+    addNamedSource('/part.dart', 'part of lib;');
+    Source source = addSource(r'''
+library lib;
+part 'part.dart';
+part 'part.dart';
+''');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [CompileTimeErrorCode.DUPLICATE_PART]);
+    verify([source]);
+  }
+
   void test_exportInternalLibrary() {
     Source source = addSource("export 'dart:_interceptors';");
     computeLibrarySourceErrors(source);
     assertErrors(source, [CompileTimeErrorCode.EXPORT_INTERNAL_LIBRARY]);
     verify([source]);
   }
 
   void test_exportOfNonLibrary() {
     Source source = addSource(r'''
 library L;
@@ skipping 4427 matching lines @@
         source, [CompileTimeErrorCode.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR]);
     verify([source]);
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "");
     _check_wrongNumberOfParametersForOperator(name, "a, b");
   }
 }