Fix constant evaluation dependency analysis for const external factories.

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

Index: pkg/analyzer/test/src/task/dart_test.dart
diff --git a/pkg/analyzer/test/src/task/dart_test.dart b/pkg/analyzer/test/src/task/dart_test.dart
index 8e3882c49fc4860c1ea6349cd0d631db6df3c7c1..a3cf7764e632bc052b6a391a75fc84ff97e0f26b 100644
--- a/pkg/analyzer/test/src/task/dart_test.dart
+++ b/pkg/analyzer/test/src/task/dart_test.dart
@@ -1382,6 +1382,20 @@ const y = 1;
     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', '''