anchor.diff:    FindNode findNode;
   
     Source source;
  +
  +  bool get useFixBuilder;
   
     Future<void> analyze(String content) async {
       var sourcePath = convertPath('/home/test/lib/test.dart');
       newFile(sourcePath, content: content);
       var listener = new TestMigrationListener();
       var migration = NullabilityMigration(listener,
  -        instrumentation: _InstrumentationClient(this));
  +        instrumentation: _InstrumentationClient(this),
  +        useFixBuilder: useFixBuilder);
       var result = await session.getResolvedUnit(sourcePath);
       source = result.unit.declaredElement.source;
       findNode = FindNode(content, result.unit);
  
positive.code: main() {
    defineReflectiveSuite(() {
      defineReflectiveTests(_InstrumentationTest);
    });
  }
  
  class _InstrumentationClient implements NullabilityMigrationInstrumentation {
    final _InstrumentationTest test;
  
    _InstrumentationClient(this.test);
  
  
negative.code: }
  
  @reflectiveTest
  class _InstrumentationTest extends _InstrumentationTestBase {
    @override
    bool get useFixBuilder => false;
  }
  
  abstract class _InstrumentationTestBase extends AbstractContextTest {
    NullabilityNodeInfo always;
  
    final Map<TypeAnnotation, NullabilityNodeInfo> explicitTypeNullability = {};
  