Create default tasks if plugins are not being used

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.src.task.abstract_context_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 67 matching lines @@
   List<Source> newSources(Map<String, String> sourceMap) {
     List<Source> sources = <Source>[];
     sourceMap.forEach((String path, String content) {
       Source source = newSource(path, content);
       sources.add(source);
     });
     return sources;
   }
 
   void setUp() {
-    // configure TaskManager
-    {
-      EnginePlugin plugin = AnalysisEngine.instance.enginePlugin;
-      if (plugin.taskExtensionPoint == null) {
-        extensionManager.processPlugins([plugin]);

[scheglov, 2015/05/19 21:09:25]

I this case the "extensionManager" field is not used at all and can be removed.

[Brian Wilkerson, 2015/05/19 21:12:08]

Done.

I also removed two unused imports.

-      }
-    }
     // prepare AnalysisContext
     sourceFactory = new SourceFactory(<UriResolver>[
       new DartUriResolver(sdk),
       new ResourceUriResolver(resourceProvider)
     ]);
     context = createAnalysisContext();
     context.sourceFactory = sourceFactory;
     analysisDriver = context.driver;
   }
 
   void tearDown() {}
 }