Be sure to use absolute paths in intl web server tests

pkg/intl/test/date_time_format_http_request_test.dart

Index: pkg/intl/test/date_time_format_http_request_test.dart
===================================================================
--- pkg/intl/test/date_time_format_http_request_test.dart	(revision 12664)
+++ pkg/intl/test/date_time_format_http_request_test.dart	(working copy)
@@ -11,9 +11,11 @@
 // 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.
+// 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');