Issue 25050. Fix for NPE in ConstantEvaluationEngine when duplicate fields.

pkg/analyzer/test/src/task/dart_test.dart

 // Copyright (c) 2015, 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 test.src.task.dart_test;
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 4392 matching lines @@
 ''');
     LibrarySpecificUnit target = new LibrarySpecificUnit(source, source);
     computeResult(target, VERIFY_ERRORS, matcher: isVerifyUnitTask);
     // validate
     _fillErrorListener(VERIFY_ERRORS);
     errorListener.assertErrorsWithCodes(<ErrorCode>[
       CompileTimeErrorCode.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE
     ]);
   }
 
+  test_perform_ConstantValidator_duplicateFields() {
+    Source source = newSource(
+        '/test.dart',
+        '''
+class Test {
+  final int x = 1;
+  final int x = 2;
+  const Test();
+}
+
+main() {
+  const Test();
+}
+''');
+    LibrarySpecificUnit target = new LibrarySpecificUnit(source, source);
+    computeResult(target, VERIFY_ERRORS, matcher: isVerifyUnitTask);
+    // validate
+    _fillErrorListener(VERIFY_ERRORS);
+    errorListener.assertNoErrors();
+  }
+
   test_perform_directiveError() {
     Source source = newSource(
         '/test.dart',
         '''
 import 'no-such-file.dart';
 ''');
     LibrarySpecificUnit target = new LibrarySpecificUnit(source, source);
     computeResult(target, VERIFY_ERRORS, matcher: isVerifyUnitTask);
     // validate
     _fillErrorListener(VERIFY_ERRORS);
@@ skipping 141 matching lines @@
   /**
    * Fill [errorListener] with [result] errors in the current [task].
    */
   void _fillErrorListener(ResultDescriptor<List<AnalysisError>> result) {
     List<AnalysisError> errors = task.outputs[result];
     expect(errors, isNotNull, reason: result.name);
     errorListener = new GatheringErrorListener();
     errorListener.addAll(errors);
   }
 }