Add Driver.useSingleContextManager to turn on single context manager.

pkg/analyzer/lib/plugin/resolver_provider.dart

Index: pkg/analyzer/lib/plugin/resolver_provider.dart
diff --git a/pkg/analyzer/lib/plugin/resolver_provider.dart b/pkg/analyzer/lib/plugin/resolver_provider.dart
index 117237236a678cf17a46d51fb957a5ffd9381e1d..ed0e3667e96525ee66013f986887396bd9cc56cb 100644
--- a/pkg/analyzer/lib/plugin/resolver_provider.dart
+++ b/pkg/analyzer/lib/plugin/resolver_provider.dart
@@ -9,6 +9,10 @@ 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);