Fix test failures on the bots

pkg/analyzer/test/src/context/abstract_context.dart

Index: pkg/analyzer/test/src/context/abstract_context.dart
diff --git a/pkg/analyzer/test/src/context/abstract_context.dart b/pkg/analyzer/test/src/context/abstract_context.dart
index 135fd426a8a7e4681de402efe8ec2d0261c7b5f4..7ba1ba5f643e113a9803aa8f0f09cd5f3c74cf5f 100644
--- a/pkg/analyzer/test/src/context/abstract_context.dart
+++ b/pkg/analyzer/test/src/context/abstract_context.dart
@@ -15,6 +15,7 @@ import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/task/driver.dart';
 import 'package:analyzer/task/model.dart';
+import 'package:plugin/manager.dart';
 import 'package:unittest/unittest.dart';
 
 import 'mock_sdk.dart';
@@ -124,6 +125,8 @@ class AbstractContextTest {
   }
 
   void setUp() {
+    ExtensionManager extensionManager = new ExtensionManager();
+    extensionManager.processPlugins(AnalysisEngine.instance.supportedPlugins);
     prepareAnalysisContext();
   }