Run analyzer_cli asynchronously.

pkg/analyzer_cli/test/sdk_ext_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.
 
 /// Test that sdk extensions are properly detected in various scenarios.
 library analyzer_cli.test.sdk_ext;
 
 import 'dart:io';
 
 import 'package:analyzer/src/dart/sdk/sdk.dart';
@@ skipping 22 matching lines @@
     });
     tearDown(() {
       outSink = savedOutSink;
       errorSink = savedErrorSink;
       exitCode = savedExitCode;
       exitHandler = savedExitHandler;
     });
 
     test('.packages file specified', () async {
       String testDir = path.join(testDirectory, 'data', 'packages_file');
-      Driver driver = new Driver()
-        ..start([
-          '--packages',
-          path.join(testDir, '_packages'),
-          path.join(testDir, 'sdk_ext_user.dart')
-        ]);
+      Driver driver = new Driver();
+      await driver.start([
+        '--packages',
+        path.join(testDir, '_packages'),
+        path.join(testDir, 'sdk_ext_user.dart')
+      ]);
 
       DartSdk sdk = driver.sdk;
       expect(sdk, new isInstanceOf<FolderBasedDartSdk>());
       expect((sdk as FolderBasedDartSdk).useSummary, isFalse);
 
       expect(exitCode, 0);
     });
   });
 }