Add support for built-in plugins

pkg/analysis_server/test/domain_completion_test.dart

Index: pkg/analysis_server/test/domain_completion_test.dart
diff --git a/pkg/analysis_server/test/domain_completion_test.dart b/pkg/analysis_server/test/domain_completion_test.dart
index e477ab2f12a1d726f1dcffd029a65d33d05e1b59..dc2bb574684436d032438c7b70358699438f9a45 100644
--- a/pkg/analysis_server/test/domain_completion_test.dart
+++ b/pkg/analysis_server/test/domain_completion_test.dart
@@ -708,7 +708,7 @@ class B extends A {m() {^}}
         ^
       }
     ''');
-    PluginInfo info = new PluginInfo('a', 'b', 'c', null, null);
+    PluginInfo info = new DiscoveredPluginInfo('a', 'b', 'c', null, null);
     plugin.CompletionGetSuggestionsResult result =
         new plugin.CompletionGetSuggestionsResult(
             1, 2, <plugin.CompletionSuggestion>[