Switch the default from non-driver to driver in several tests

pkg/analysis_server/test/abstract_single_unit.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.
 
 library test.services.src.index.abstract_single_file;
 
 import 'dart:async';
+
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/dart/ast/utilities.dart';
 import 'package:analyzer/src/dart/error/hint_codes.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test/test.dart';
 
 import 'abstract_context.dart';
@@ skipping 96 matching lines @@
     } else {
       testUnit = await resolveLibraryUnit(testSource);
       if (verifyNoTestUnitErrors) {
         expect(context.getErrors(testSource).errors, isEmpty);
       }
     }
     testUnitElement = testUnit.element;
     testLibraryElement = testUnitElement.library;
   }
 }