Add context information to driver and add hooks for plugin management

pkg/front_end/lib/src/incremental_resolved_ast_generator_impl.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/dart/analysis/byte_store.dart';
@@ skipping 112 matching lines @@
         new SourceFactory([new DartUriResolver(dartSdk)]);
 
     var sourceFactory = new _SourceFactoryProxy(dartSdk, _fileRepository);
     var analysisOptions = new AnalysisOptionsImpl();
     _driver = new driver.AnalysisDriver(
         _scheduler,
         performanceLog,
         _resourceProvider,
         byteStore,
         fileContentOverlay,
-        'front_end',
+        null,
         sourceFactory,
         analysisOptions,
         sdkBundle: sdkBundle);
     _isInitialized = true;
   }
 
   @override
   void invalidate(String path) {
     throw new UnimplementedError();
   }
@@ skipping 149 matching lines @@
 class _SourceProxy extends BasicSource {
   @override
   final String fullName;
 
   _SourceProxy(Uri uri, this.fullName) : super(uri);
 
   int get modificationStamp => 0;
 
   noSuchMethod(Invocation invocation) => unimplemented();
 }