Optimize more megamorphic dispatch sites

pkg/analyzer/lib/instrumentation/instrumentation.dart

 // Copyright (c) 2014, 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.instrumentation;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/task/model.dart';
@@ skipping 302 matching lines @@
     buffer.write(field.substring(start));
   }
 
   /**
    * Return the result of joining the values of the given fields, escaping the
    * separator character by doubling it.
    */
   String _join(List<String> fields) {
     StringBuffer buffer = new StringBuffer();
     buffer.write(_timestamp);
-    for (String field in fields) {
+    int length = fields.length;
+    for (int i = 0; i < length; i++) {
       buffer.write(':');
-      _escape(buffer, field);
+      _escape(buffer, fields[i]);
     }
     return buffer.toString();
   }
 
   /**
    * Log the given message with the given tag.
    */
   void _log(String tag, String message) {
     if (_instrumentationServer != null) {
       _instrumentationServer.log(_join([tag, message]));
@@ skipping 33 matching lines @@
     }
   }
 
   @override
   Future shutdown() async {
     for (InstrumentationServer server in _servers) {
       await server.shutdown();
     }
   }
 }