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 6c4db412700a266f8c7009da6a0a09628da3ede1..9765c64c6bacad1545663368f482a5b96a5b5cae 100644
--- a/runtime/bin/http_impl.dart
+++ b/runtime/bin/http_impl.dart
@@ -91,8 +91,7 @@ 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);
   }
 
@@ -111,8 +110,7 @@ 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);
   }