Handle session logs and add event subset selection

pkg/analysis_server/tool/instrumentation/log_viewer.dart

 // Copyright (c) 2016, 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:io' as io;
 
 import 'package:args/args.dart';
 
 import 'log/log.dart';
 import 'server.dart';
@@ skipping 100 matching lines @@
     List<String> lines;
     try {
       lines = logFile.readAsLinesSync();
     } catch (exception, stackTrace) {
       printUsage(parser,
           error: 'Could not read file "$fileName":',
           exception: exception,
           stackTrace: stackTrace);
       return;
     }
+    print('Log file contains ${lines.length} lines');
 
     InstrumentationLog log =
         new InstrumentationLog(<String>[logFile.path], lines);
     WebServer server = new WebServer(log);
     server.serveHttp(port);
     print('logViewer is listening on http://localhost:$port/log');
   }
 }