Reapply "Add support for timezone offset and timezone name."

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
diff --git a/lib/compiler/implementation/lib/js_helper.dart b/lib/compiler/implementation/lib/js_helper.dart
index 9995935b32a80b604d564c98671d256450b2a224..c46f2f42b49b0d96f54c624576d92c45da2c46ba 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -328,6 +328,19 @@ class Primitives {
     return JS('String', @'String.fromCharCode.apply(#, #)', null, charCodes);
   }
 
+  static String getTimeZoneName(receiver) {
+    // When calling toString on a Date it will emit the timezone in parenthesis.
+    // Example: "Wed May 16 2012 21:13:00 GMT+0200 (CEST)".
+    // We extract this name using a regexp.
+    var d = lazyAsJsDate(receiver);
+    return JS('String', @'/\((.*)\)/.exec(#.toString())[1]', d);
+  }
+
+  static int getTimeZoneOffsetInMinutes(receiver) {
+    // Note that JS and Dart disagree on the sign of the offset.
+    return -JS('int', @'#.getTimezoneOffset()', lazyAsJsDate(receiver));
+  }
+
   static valueFromDecomposedDate(years, month, day, hours, minutes, seconds,
                                  milliseconds, isUtc) {
     checkInt(years);