Add Driver.useSingleContextManager to turn on single context manager.

pkg/analyzer/lib/plugin/resolver_provider.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.plugin.resolver_provider;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * A function that will return a [UriResolver] that can be used to resolve a
- * specific kind of URI within the analysis context rooted at the given folder.
+ * specific kind of URI within the analysis context rooted at the given
+ * [folder].
+ *
+ * The given [folder] may be `null` if analysis is performed in an environment
+ * where URI resolution is always the same in any folder.
  */
 typedef UriResolver ResolverProvider(Folder folder);