Fix test failures on the bots

pkg/analyzer/tool/task_dependency_graph/generate.dart

Index: pkg/analyzer/tool/task_dependency_graph/generate.dart
diff --git a/pkg/analyzer/tool/task_dependency_graph/generate.dart b/pkg/analyzer/tool/task_dependency_graph/generate.dart
index daac1029e21ad5db3c37a4cd1564f0a4267cec0e..5f69b36b14e039695910f809393f2188f347835c 100644
--- a/pkg/analyzer/tool/task_dependency_graph/generate.dart
+++ b/pkg/analyzer/tool/task_dependency_graph/generate.dart
@@ -34,6 +34,7 @@ import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:path/path.dart' as path;
 import 'package:path/path.dart';
+import 'package:plugin/manager.dart';
 
 /**
  * Generate the target .dot file.
@@ -120,6 +121,9 @@ class Driver {
    * Generate the task dependency graph and return it as a [String].
    */
   String generateFileContents() {
+    ExtensionManager extensionManager = new ExtensionManager();
+    extensionManager.processPlugins(AnalysisEngine.instance.supportedPlugins);
+
     List<String> lines = <String>[];
     resourceProvider = PhysicalResourceProvider.INSTANCE;
     DartSdk sdk = DirectoryBasedDartSdk.defaultSdk;