Add support for getting the sessionId from instrumentation and sending it to the client

pkg/analyzer/lib/instrumentation/file_instrumentation.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 analyzer.instrumentation.file_instrumentation;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:analyzer/instrumentation/instrumentation.dart';
 
 /**
  * An [InstrumentationServer] that writes to a file.
  */
 class FileInstrumentationServer implements InstrumentationServer {
   IOSink _sink;
 
   FileInstrumentationServer(String path) {
     File file = new File(path);
     _sink = file.openWrite();
   }
 
   @override
+  String get sessionId => '';
+
+  @override
   void log(String message) {
     _sink.writeln(message);
   }
 
   @override
   void logWithPriority(String message) {
     log(message);
   }
 
   @override
   Future shutdown() async {
     await _sink.close();
     _sink = null;
   }
 }