Run analyzer_cli asynchronously.

pkg/analyzer_cli/test/super_mixin_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 analyzer_cli.test.super_mixin;
 
 import 'dart:io';
 
 import 'package:analyzer_cli/src/driver.dart' show Driver, errorSink, outSink;
 import 'package:path/path.dart' as path;
@@ skipping 20 matching lines @@
       errorSink = new StringBuffer();
     });
     tearDown(() {
       outSink = savedOutSink;
       errorSink = savedErrorSink;
       exitCode = savedExitCode;
     });
 
     test('produces errors when option absent', () async {
       var testPath = path.join(testDirectory, 'data/super_mixin_example.dart');
-      new Driver().start([testPath]);
+      await new Driver().start([testPath]);
 
       expect(exitCode, 3);
       var stdout = outSink.toString();
       expect(
           stdout,
           contains(
               "[error] The class 'C' can't be used as a mixin because it extends a class other than Object"));
       expect(
           stdout,
           contains(
               "[error] The class 'C' can't be used as a mixin because it references 'super'"));
       expect(stdout, contains('2 errors found.'));
       expect(errorSink.toString(), '');
     });
 
     test('produces no errors when option present', () async {
       var testPath = path.join(testDirectory, 'data/super_mixin_example.dart');
-      new Driver().start(['--supermixin', testPath]);
+      await new Driver().start(['--supermixin', testPath]);
 
       expect(exitCode, 0);
       var stdout = outSink.toString();
       expect(stdout, contains('No issues found'));
     });
   });
 }