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

pkg/analyzer/lib/src/generated/constant.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 engine.constant;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/utilities_general.dart';
@@ skipping 686 matching lines @@
     }
     HashMap<String, DartObjectImpl> fieldMap =
         new HashMap<String, DartObjectImpl>();
     // Start with final fields that are initialized at their declaration site.
     for (FieldElement field in constructor.enclosingElement.fields) {
       if ((field.isFinal || field.isConst) &&
           !field.isStatic &&
           field is ConstFieldElementImpl) {
         validator.beforeGetFieldEvaluationResult(field);
         EvaluationResultImpl evaluationResult = field.evaluationResult;
+        // It is possible that the evaluation result is null.
+        // This happens for example when we have duplicate fields.
+        // class Test {final x = 1; final x = 2; const Test();}
+        if (evaluationResult == null) {
+          continue;
+        }
+        // Match the value and the type.
         DartType fieldType =
             FieldMember.from(field, constructor.returnType).type;
         DartObjectImpl fieldValue = evaluationResult.value;
         if (fieldValue != null && !runtimeTypeMatch(fieldValue, fieldType)) {
           errorReporter.reportErrorForNode(
               CheckedModeCompileTimeErrorCode.CONST_CONSTRUCTOR_FIELD_TYPE_MISMATCH,
               node,
               [fieldValue.type, field.name, fieldType]);
         }
-        fieldMap[field.name] = evaluationResult.value;
+        fieldMap[field.name] = fieldValue;
       }
     }
     // Now evaluate the constructor declaration.
     HashMap<String, DartObjectImpl> parameterMap =
         new HashMap<String, DartObjectImpl>();
     List<ParameterElement> parameters = constructor.parameters;
     int parameterCount = parameters.length;
     for (int i = 0; i < parameterCount; i++) {
       ParameterElement parameter = parameters[i];
       ParameterElement baseParameter = parameter;
@@ skipping 4833 matching lines @@
       return BoolState.from(_element == rightElement);
     } else if (rightOperand is DynamicState) {
       return BoolState.UNKNOWN_VALUE;
     }
     return BoolState.FALSE_STATE;
   }
 
   @override
   String toString() => _element == null ? "-unknown-" : _element.name;
 }