Add support for built-in plugins

pkg/analysis_server/test/edit/assists_test.dart

Index: pkg/analysis_server/test/edit/assists_test.dart
diff --git a/pkg/analysis_server/test/edit/assists_test.dart b/pkg/analysis_server/test/edit/assists_test.dart
index c8fae4f50f9ea475b4239c9456195ceef760009b..dce7822f6e329e8e8d694e7ae85615cea4cde7cb 100644
--- a/pkg/analysis_server/test/edit/assists_test.dart
+++ b/pkg/analysis_server/test/edit/assists_test.dart
@@ -51,7 +51,7 @@ class AssistsTest extends AbstractAnalysisTest {
   }
 
   test_fromPlugins() async {
-    PluginInfo info = new PluginInfo('a', 'b', 'c', null, null);
+    PluginInfo info = new DiscoveredPluginInfo('a', 'b', 'c', null, null);
     String message = 'From a plugin';
     plugin.PrioritizedSourceChange change = new plugin.PrioritizedSourceChange(
         5,