Fix NPE when computing dependencies of enum constants

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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.constant;
 
 import 'dart:collection';
@@ skipping 400 matching lines @@
     ReferenceFinder referenceFinder = new ReferenceFinder(callback);
     if (constant is ParameterElement) {
       if (constant.initializer != null) {
         Expression defaultValue =
             (constant as ConstVariableElement).constantInitializer;
         if (defaultValue != null) {
           defaultValue.accept(referenceFinder);
         }
       }
     } else if (constant is PotentiallyConstVariableElement) {
-      constant.constantInitializer.accept(referenceFinder);
+      Expression initializer = constant.constantInitializer;
+      if (initializer != null) {
+        initializer.accept(referenceFinder);
+      }
     } else if (constant is ConstructorElementImpl) {
       constant.isCycleFree = false;
       ConstructorElement redirectedConstructor =
           getConstRedirectedConstructor(constant);
       if (redirectedConstructor != null) {
         ConstructorElement redirectedConstructorBase =
             ConstantEvaluationEngine._getConstructorBase(redirectedConstructor);
         callback(redirectedConstructorBase);
         return;
       } else if (constant.isFactory) {
@@ skipping 4785 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;
 }