anchor.diff:      await resolveTestUnit('''
   import 'my_file.dart';
   ''');
  -    AnalysisError error = _findErrorToFix();
  +    AnalysisError error = await _findErrorToFix();
       fix = await _assertHasFix(DartFixKind.CREATE_FILE, error);
       change = fix.change;
       // validate change
  
positive.code:   }
  
    assertNoFix(FixKind kind) async {
      AnalysisError error = _findErrorToFix();
      List<Fix> fixes = await _computeFixes(error);
      for (Fix fix in fixes) {
        if (fix.kind == kind) {
  
negative.code:     await resolveTestUnit('''
  import 'a/bb/my_lib.dart';
  ''');
      AnalysisError error = await _findErrorToFix();
      fix = await _assertHasFix(DartFixKind.CREATE_FILE, error);
      change = fix.change;
      // validate change
  