Fix constant evaluation dependency analysis for const external factories.

pkg/analyzer/test/src/task/dart_test.dart

 // Copyright (c) 2015, 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 test.src.task.dart_test;
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 1364 matching lines @@
     EvaluationResultImpl evaluationResult = _computeTopLevelVariableConstValue(
         'x', '''
 const x = y + 1;
 const y = 1;
 ''');
     expect(evaluationResult, isNotNull);
     expect(evaluationResult.value, isNotNull);
     expect(evaluationResult.value.intValue, 2);
   }
 
+  test_external_const_factory() {
+    EvaluationResultImpl evaluationResult = _computeTopLevelVariableConstValue(
+        'x', '''
+const x = const C.foo();
+
+class C extends B {
+  external const factory C.foo();
+}
+
+class B {}
+''');
+    expect(evaluationResult, isNotNull);
+  }
+
   test_simple_constant() {
     EvaluationResultImpl evaluationResult = _computeTopLevelVariableConstValue(
         'x', '''
 const x = 1;
 ''');
     expect(evaluationResult, isNotNull);
     expect(evaluationResult.value, isNotNull);
     expect(evaluationResult.value.intValue, 1);
   }
 
@@ skipping 1004 matching lines @@
   /**
    * Fill [errorListener] with [result] errors in the current [task].
    */
   void _fillErrorListener(ResultDescriptor<List<AnalysisError>> result) {
     List<AnalysisError> errors = task.outputs[result];
     expect(errors, isNotNull, reason: result.name);
     errorListener = new GatheringErrorListener();
     errorListener.addAll(errors);
   }
 }