Patch to work around exception until the root cause is found

pkg/analyzer/lib/src/dart/constant/evaluation.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.constant.evaluation;
 
 import 'dart:collection';
 
 import 'package:analyzer/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 329 matching lines @@
         // so it depends on the variable.
         callback(element.variable);
       } else if (element is ConstructorElementImpl) {
         // The annotation is a constructor invocation, so it depends on the
         // constructor.
         callback(element);
       } else {
         // This could happen in the event of invalid code.  The error will be
         // reported at constant evaluation time.
       }
-      if (constNode.arguments != null) {
+      if (constNode == null) {
+        // We cannot determine what element the annotation is on, nor the offset
+        // of the annotation, so there's not a lot of information in this
+        // message, but it's better than getting an exception.
+        AnalysisEngine.instance.logger.logInformation(
+            'No annotationAst for $constant in ${constant.compilationUnit}');
+      } else if (constNode.arguments != null) {
         constNode.arguments.accept(referenceFinder);
       }
     } else if (constant is VariableElement) {
       // constant is a VariableElement but not a VariableElementImpl.  This can
       // happen sometimes in the case of invalid user code (for example, a
       // constant expression that refers to a non-static field inside a generic
       // class will wind up referring to a FieldMember).  So just don't bother
       // computing any dependencies.
     } else {
       // Should not happen.
@@ skipping 1620 matching lines @@
   }
 
   @override
   String toString() {
     if (value == null) {
       return "error";
     }
     return value.toString();
   }
 }