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

lib/compiler/implementation/lib/js_helper.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('js_helper');
 
 #import('coreimpl.dart');
 
 #source('constant_map.dart');
 #source('native_helper.dart');
@@ skipping 310 matching lines @@
 
   static num dateNow() => JS('num', @'Date.now()');
 
   static String stringFromCharCodes(charCodes) {
     for (var i in charCodes) {
       if (i is !int) throw new IllegalArgumentException(i);
     }
     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);
     checkInt(month);
     if (month < 1 || 12 < month) throw new IllegalArgumentException(month);
     checkInt(day);
     if (day < 1 || 31 < day) throw new IllegalArgumentException(day);
     checkInt(hours);
     if (hours < 0 || 24 < hours) throw new IllegalArgumentException(hours);
     checkInt(minutes);
@@ skipping 549 matching lines @@
   if (JS('bool', '!!#[#]', value, property)) return value;
   propertyTypeError(value, property);
 }
 
 listSuperNativeTypeCheck(value, property) {
   if (value === null) return value;
   if (value is List) return value;
   if (JS('bool', '#[#]()', value, property)) return value;
   propertyTypeError(value, property);
 }