Make invoking start_web_server use an absolute path to accomodate incremental dartium builds

pkg/intl/test/date_time_format_http_request_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // or details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Test date formatting and parsing using locale data read via an http request
  * to a server.
  */
 
 // TODO(alanknight): It would be nice if there were a more general facility
 // than this and it could be replaced, possibly the things Graham is working on.
 // The following magical comments will cause the test code run via test.py
 // to add an extra command starting up a web server that will serve the
 // files we are looking for. "dart" will be replaced by the current dart
 // VM, and in arguments, $dartDir will be replaced by the current Dart
 // root directory.
 // ExtraCommand=dart
-// ExtraCommandArgs=pkg/intl/test/start_web_server.dart
+// ExtraCommandArgs=$dartDir/pkg/intl/test/start_web_server.dart
 #library('date_time_format_http_request_test');
 
 #import('../lib/date_format.dart');
 #import('../lib/date_symbol_data_http_request.dart');
 #import('date_time_format_test_core.dart');
 #import('dart:html');
 
 var url = "http://localhost:8000/dates/";
 
 main() {
   // Initialize one locale just so we know what the list is.
   initializeDateFormatting("en_US", url).then(runEverything);
 }
 
 void runEverything(_) {
   // Initialize all locales and wait for them to finish before running tests.
   var futures = DateFormat.allLocalesWithSymbols().map(
       (locale) => initializeDateFormatting(locale, url));
   Futures.wait(futures).then((results) {
     runDateTests();
     shutDown();});
 }
 
 void shutDown() {
   // The tiny web server knows to use this request as a cue to terminate.
   var source = '$url/terminate';
   var request = new HttpRequest.get(source, (_) {});
 }