Fix constant evaluation dependency analysis for const external factories.

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 409 matching lines @@
       constant.constantInitializer.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) {
+        // Factory constructor, but getConstRedirectedConstructor returned
+        // null.  This can happen if we're visiting one of the special external
+        // const factory constructors in the SDK, or if the code contains
+        // errors (such as delegating to a non-const constructor, or delegating
+        // to a constructor that can't be resolved).  In any of these cases,
+        // we'll evaluate calls to this constructor without having to refer to
+        // any other constants.  So we don't need to report any dependencies.
+        return;
       }
       bool superInvocationFound = false;
       List<ConstructorInitializer> initializers = constant.constantInitializers;
       for (ConstructorInitializer initializer in initializers) {
         if (initializer is SuperConstructorInvocation) {
           superInvocationFound = true;
         }
         initializer.accept(referenceFinder);
       }
       if (!superInvocationFound) {
@@ skipping 4766 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;
 }