Fix a recent regression with nested contexts

pkg/analysis_server/test/analysis_abstract.dart

Index: pkg/analysis_server/test/analysis_abstract.dart
diff --git a/pkg/analysis_server/test/analysis_abstract.dart b/pkg/analysis_server/test/analysis_abstract.dart
index 53f04afd31955a4fabe8002e2e05828b7c1f5ed7..8fbfbc85e14b613c7dc0f1cda3d0233cda36bb03 100644
--- a/pkg/analysis_server/test/analysis_abstract.dart
+++ b/pkg/analysis_server/test/analysis_abstract.dart
@@ -299,6 +299,12 @@ class TestPluginManager implements PluginManager {
   }
 
   @override
+  List<PluginInfo> get plugins {
+    fail('Unexpected invocation of plugins');
+    return null;
+  }
+
+  @override
   ResourceProvider get resourceProvider {
     fail('Unexpected invocation of resourceProvider');
     return null;