Use language or userlanguage (IE) depending on what's defined in the browser.

sdk/lib/html/dart2js/html_dart2js.dart

 library html;
 
 import 'dart:html_common';
 import 'dart:isolate';
 import 'dart:json';
 import 'dart:svg' as svg;
 import 'dart:web_audio' as web_audio;
 // 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.
@@ skipping 5999 matching lines @@
 
   /// @domName DirectoryReaderSync.readEntries; @docsEditable true
   @Returns('_EntryArraySync') @Creates('_EntryArraySync')
   List<EntrySync> readEntries() native;
 }
 // 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.
 
 
-/**
- * Represents an HTML <div> element.
- *
- * The [DivElement] is a generic container for content and does not have any
- * special significance. It is functionally similar to [SpanElement].
- *
- * The [DivElement] is a block-level element, as opposed to [SpanElement],
- * which is an inline-level element.
- *
- * Example usage:
- *
- *     DivElement div = new DivElement();
- *     div.text = 'Here's my new DivElem
- *     document.body.elements.add(elem);
- *
- * See also:
- *
- * * [HTML <div> element](http://www.w3.org/TR/html-markup/div.html) from W3C.
- * * [Block-level element](http://www.w3.org/TR/CSS2/visuren.html#block-boxes) from W3C.
- * * [Inline-level element](http://www.w3.org/TR/CSS2/visuren.html#inline-boxes) from W3C.
- */
 /// @domName HTMLDivElement; @docsEditable true
 class DivElement extends Element implements Element native "*HTMLDivElement" {
 
   factory DivElement() => document.$dom_createElement("div");
 }
 // 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.
 
 
@@ skipping 18 matching lines @@
   /// @domName Document.body; @docsEditable true
   @JSName('body')
   Element $dom_body;
 
   /// @domName Document.charset; @docsEditable true
   String charset;
 
   /// @domName Document.cookie; @docsEditable true
   String cookie;
 
-  /// Returns the [Window] associated with the document.
   /// @domName Document.defaultView; @docsEditable true
   Window get window => _convertNativeToDart_Window(this._window);
   @JSName('defaultView')
   @Creates('LocalWindow|=Object') @Returns('LocalWindow|=Object')
   final dynamic _window;
 
   /// @domName Document.documentElement; @docsEditable true
   final Element documentElement;
 
   /// @domName Document.domain; @docsEditable true
@@ skipping 6699 matching lines @@
   final int totalJSHeapSize;
 
   /// @domName MemoryInfo.usedJSHeapSize; @docsEditable true
   final int usedJSHeapSize;
 }
 // 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.
 
 
-/**
- * An HTML <menu> element.
- *
- * A <menu> element represents an unordered list of menu commands.
- *
- * See also:
- *
- *  * [Menu Element](https://developer.mozilla.org/en-US/docs/HTML/Element/menu) from MDN.
- *  * [Menu Element](http://www.w3.org/TR/html5/the-menu-element.html#the-menu-element) from the W3C.
- */
 /// @domName HTMLMenuElement; @docsEditable true
 class MenuElement extends Element implements Element native "*HTMLMenuElement" {
 
   factory MenuElement() => document.$dom_createElement("menu");
 }
 // 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.
 
 
@@ skipping 576 matching lines @@
   Node setNamedItemNS(Node node) native;
 }
 // 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.
 
 
 /// @domName Navigator; @docsEditable true
 class Navigator native "*Navigator" {
 
+  /// @domName Navigator.language; @docsEditable true
+  String get language => JS('String', '#.language || #.userLanguage', this,
+      this);
+  
   /// @domName Navigator.appCodeName; @docsEditable true
   final String appCodeName;
 
   /// @domName Navigator.appName; @docsEditable true
   final String appName;
 
   /// @domName Navigator.appVersion; @docsEditable true
   final String appVersion;
 
   /// @domName Navigator.cookieEnabled; @docsEditable true
   final bool cookieEnabled;
 
   /// @domName Navigator.geolocation; @docsEditable true
   final Geolocation geolocation;
 
-  /// @domName Navigator.language; @docsEditable true
-  final String language;
-
   /// @domName Navigator.mimeTypes; @docsEditable true
   final DOMMimeTypeArray mimeTypes;
 
   /// @domName Navigator.onLine; @docsEditable true
   final bool onLine;
 
   /// @domName Navigator.platform; @docsEditable true
   final String platform;
 
   /// @domName Navigator.plugins; @docsEditable true
@@ skipping 35 matching lines @@
       return;
     }
     var options_2 = convertDartToNative_Dictionary(options);
     _webkitGetUserMedia_2(options_2, successCallback);
     return;
   }
   @JSName('webkitGetUserMedia')
   void _webkitGetUserMedia_1(options, NavigatorUserMediaSuccessCallback successCallback, NavigatorUserMediaErrorCallback errorCallback) native;
   @JSName('webkitGetUserMedia')
   void _webkitGetUserMedia_2(options, NavigatorUserMediaSuccessCallback successCallback) native;
+
 }
 // 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.
 
 
 /// @domName NavigatorUserMediaError; @docsEditable true
 class NavigatorUserMediaError native "*NavigatorUserMediaError" {
 
   static const int PERMISSION_DENIED = 1;
@@ skipping 11591 matching lines @@
     if (length < 0) throw new ArgumentError('length');
     if (start < 0) throw new RangeError.value(start);
     int end = start + length;
     if (end > a.length) throw new RangeError.value(end);
     for (int i = start; i < end; i++) {
       accumulator.add(a[i]);
     }
     return accumulator;
   }
 }