Update PluginWatcher to be more efficient

pkg/analysis_server/test/src/plugin/plugin_watcher_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:typed_data';
 
 import 'package:analysis_server/src/plugin/plugin_manager.dart';
 import 'package:analysis_server/src/plugin/plugin_watcher.dart';
 import 'package:analyzer/context/context_root.dart';
@@ skipping 48 matching lines @@
     //
     resourceProvider.newFile(
         resourceProvider.convertPath('/pkg2/lib/pkg2.dart'), '');
     resourceProvider.newFile(
         resourceProvider.convertPath('/pkg2/pubspec.yaml'), 'name: pkg2');
     resourceProvider.newFile(
         resourceProvider
             .convertPath('/pkg2/tools/analysis_plugin/bin/plugin.dart'),
         '');
     await driver.computeResult('package:pkg2/pk2.dart');
+    //
+    // Wait until the timer associated with the driver's FileSystemState is
+    // guaranteed to have expired and the list of changed files will have been
+    // delivered.
+    //
+    await new Future.delayed(new Duration(seconds: 1));
     expect(manager.addedContextRoots, hasLength(1));
   }
 
   void test_creation() {
     expect(watcher.resourceProvider, resourceProvider);
     expect(watcher.manager, manager);
   }
 
   test_removedDriver() {
     String pkg1Path = resourceProvider.convertPath('/pkg1');
@@ skipping 62 matching lines @@
     return null;
   }
 
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   void removedContextRoot(ContextRoot contextRoot) {
     removedContextRoots.add(contextRoot);
   }
 }