Deprecate use of timezones.

runtime/bin/http_impl.dart

Index: runtime/bin/http_impl.dart
diff --git a/runtime/bin/http_impl.dart b/runtime/bin/http_impl.dart
index 05c0a8f3907ac8e08d5e94d19a022374a5f9430a..3e93389095d0e8909fdc3433207b33c6458392f1 100644
--- a/runtime/bin/http_impl.dart
+++ b/runtime/bin/http_impl.dart
@@ -90,8 +90,7 @@ class _HttpHeaders implements HttpHeaders {
   void set date(Date date) {
     _checkMutable();
     // Format "Date" header with date in Greenwich Mean Time (GMT).
-    String formatted =
-        _HttpUtils.formatDate(expires.changeTimeZone(new TimeZone.utc()));
+    String formatted = _HttpUtils.formatDate(expires.toUtc());
     _set("date", formatted);
   }
 
@@ -110,8 +109,7 @@ class _HttpHeaders implements HttpHeaders {
   void set expires(Date expires) {
     _checkMutable();
     // Format "Expires" header with date in Greenwich Mean Time (GMT).
-    String formatted =
-        _HttpUtils.formatDate(expires.changeTimeZone(new TimeZone.utc()));
+    String formatted = _HttpUtils.formatDate(expires.toUtc());
     _set("expires", formatted);
   }