Ensure that nulls aren't added to constant eval dependency graph.

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 1180 matching lines @@
         } else {
           // This may happen for invalid code (e.g. failing to pass arguments
           // to an annotation which references a const constructor).  The error
           // is detected elsewhere, so just silently ignore it here.
           elementAnnotation.evaluationResult =
               new EvaluationResultImpl.con1(null);
         }
       }
     } else {
       // Should not happen.
+      assert(false);
       AnalysisEngine.instance.logger.logError(
           "Constant value computer trying to compute the value of a node which is not a VariableDeclaration, InstanceCreationExpression, FormalParameter, or ConstructorDeclaration");
       return;
     }
   }
 
   /**
    * Generate an error indicating that the given [constNode] is not a valid
    * compile-time constant because it references at least one of the constants
    * in the given [cycle], each of which directly or indirectly references the
@@ skipping 3703 matching lines @@
           ConstructorElement constructor) => _constructorDeclarationMap[
       ConstantEvaluationEngine._getConstructorBase(constructor)];
 
   @override
   Object visitInstanceCreationExpression(InstanceCreationExpression node) {
     if (node.isConst) {
       ConstructorElement constructor = node.staticElement;
       if (constructor != null) {
         ConstructorDeclaration declaration =
             findConstructorDeclaration(constructor);
-        _referenceGraph.addEdge(_source, declaration);
+        if (declaration != null) {
+          _referenceGraph.addEdge(_source, declaration);
+        }
       }
     }
     return super.visitInstanceCreationExpression(node);
   }
 
   @override
   Object visitRedirectingConstructorInvocation(
       RedirectingConstructorInvocation node) {
     super.visitRedirectingConstructorInvocation(node);
     ConstructorElement target = node.staticElement;
@@ skipping 259 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;
 }