Fix unhandle exception when access ElementAnnotationImpl.constantValue

pkg/analyzer/lib/src/dart/element/element.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.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1203 matching lines @@
    * [name] and [offset].
    */
   ConstFieldElementImpl(String name, int offset) : super(name, offset);
 
   /**
    * Initialize a newly created field element to have the given [name].
    */
   ConstFieldElementImpl.forNode(Identifier name) : super.forNode(name);
 
   @override
-  DartObject get constantValue => _result.value;
+  DartObject get constantValue => _result?.value;
 
   @override
   EvaluationResultImpl get evaluationResult => _result;
 
   @override
   void set evaluationResult(EvaluationResultImpl result) {
     this._result = result;
   }
 }
 
@@ skipping 13 matching lines @@
    * and [offset].
    */
   ConstLocalVariableElementImpl(String name, int offset) : super(name, offset);
 
   /**
    * Initialize a newly created local variable element to have the given [name].
    */
   ConstLocalVariableElementImpl.forNode(Identifier name) : super.forNode(name);
 
   @override
-  DartObject get constantValue => _result.value;
+  DartObject get constantValue => _result?.value;
 
   @override
   EvaluationResultImpl get evaluationResult => _result;
 
   @override
   void set evaluationResult(EvaluationResultImpl result) {
     this._result = result;
   }
 }
 
@@ skipping 138 matching lines @@
       : super(name, offset);
 
   /**
    * Initialize a newly created top-level variable element to have the given
    * [name].
    */
   ConstTopLevelVariableElementImpl.forNode(Identifier name)
       : super.forNode(name);
 
   @override
-  DartObject get constantValue => _result.value;
+  DartObject get constantValue => _result?.value;
 
   @override
   EvaluationResultImpl get evaluationResult => _result;
 
   @override
   void set evaluationResult(EvaluationResultImpl result) {
     this._result = result;
   }
 }
 
@@ skipping 38 matching lines @@
   DefaultFieldFormalParameterElementImpl(String name, int nameOffset)
       : super(name, nameOffset);
 
   /**
    * Initialize a newly created parameter element to have the given [name].
    */
   DefaultFieldFormalParameterElementImpl.forNode(Identifier name)
       : super.forNode(name);
 
   @override
-  DartObject get constantValue => _result.value;
+  DartObject get constantValue => _result?.value;
 
   @override
   EvaluationResultImpl get evaluationResult => _result;
 
   @override
   void set evaluationResult(EvaluationResultImpl result) {
     this._result = result;
   }
 }
 
@@ skipping 13 matching lines @@
    */
   DefaultParameterElementImpl(String name, int nameOffset)
       : super(name, nameOffset);
 
   /**
    * Initialize a newly created parameter element to have the given [name].
    */
   DefaultParameterElementImpl.forNode(Identifier name) : super.forNode(name);
 
   @override
-  DartObject get constantValue => _result.value;
+  DartObject get constantValue => _result?.value;
 
   @override
   EvaluationResultImpl get evaluationResult => _result;
 
   @override
   void set evaluationResult(EvaluationResultImpl result) {
     this._result = result;
   }
 
   @override
@@ skipping 82 matching lines @@
    */
   EvaluationResultImpl evaluationResult;
 
   /**
    * Initialize a newly created annotation. The given [compilationUnit] is the
    * compilation unit in which the annotation appears.
    */
   ElementAnnotationImpl(this.compilationUnit);
 
   @override
-  DartObject get constantValue => evaluationResult.value;
+  DartObject get constantValue => evaluationResult?.value;
 
   @override
   AnalysisContext get context => compilationUnit.library.context;
 
   @override
   bool get isDeprecated {
     if (element != null) {
       LibraryElement library = element.library;
       if (library != null && library.isDartCore) {
         if (element is ConstructorElement) {
@@ skipping 3167 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }