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

pkg/analyzer/test/instrumentation/instrumentation_test.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.test.instrumentation.instrumentation_test;
 
 import 'dart:async';
 
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ skipping 147 matching lines @@
       fail('Expected "...$message", found "$sent"');
     }
   }
 }
 
 class TestInstrumentationServer implements InstrumentationServer {
   StringBuffer normalChannel = new StringBuffer();
   StringBuffer priorityChannel = new StringBuffer();
 
   @override
+  String get sessionId => '';
+
+  @override
   void log(String message) {
     normalChannel.writeln(message);
   }
 
   @override
   void logWithPriority(String message) {
     priorityChannel.writeln(message);
   }
 
   @override
   Future shutdown() async {
     // Ignored
   }
 }