Add package config support to dart:isolate

runtime/bin/vmservice/loader.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.
 
 part of vmservice_io;
 
 _log(msg) {
   print("% $msg");
 }
 
@@ skipping 114 matching lines @@
   // p      q      r      s      t      u      v      w
      false, false, false, false, false, false, false, false,
   // x      y      z      {      |      }      ~      DEL
      false, false, false, true , true , true , false, true
 ];
 
 _parsePackagesFile(SendPort sp,
                    bool traceLoading,
                    Uri packagesFile,
                    List<int> data) {
-  var result = [];
+  // The first entry contains the location of the identified .packages file
+  // instead of a mapping.
+  var result = [packagesFile.toString(), null];
   var index = 0;
   var len = data.length;
   while (index < len) {
     var start = index;
     var char = data[index];
     if ((char == _CR) || (char == _LF)) {
       // Skipping empty lines.
       index++;
       continue;
     }
@@ skipping 280 matching lines @@
   assert(id != null);
   String resource = request[3];
   assert(resource != null);
   var uri = Uri.parse(resource);
   if (id >= 0) {
     _handleResourceRequest(sp, traceLoading, id, uri);
   } else {
     _handlePackagesRequest(sp, traceLoading, id, uri);
   }
 }