Enable getUserMedia to take constraints for its parameters.

tools/dom/templates/html/impl/impl_Navigator.darttemplate

 // 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.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
 $if DART2JS
   @DomName('Navigator.language')
   String get language => JS('String', '#.language || #.userLanguage', this,
       this);
 $endif
 
   /**
    * Gets a stream (video and or audio) from the local computer.
    *
    * Use [MediaStream.supported] to check if this is supported by the current
-   * platform.
+   * platform. The arguments `audio` and `video` default to `false` (no
+   * constraints), but will accept Maps in each parameter to specify mandatory
+   * and optional constraints for the media stream. Not passing in a map, but 
+   * passing in `true` will provide a LocalMediaStream with audio or video 
+   * capabilities, but without any additional constraints.
    *
    * Example use:
    *
-   *     window.navigator.getUserMedia(audio:true, video: true).then((stream) {
+   *     window.navigator.getUserMedia(
+   *         audio: true, 
+   *         video: {'mandatory': 

[blois, 2013/03/19 20:35:23]

It would be nice to move the details into a separate section and keep this one
as true.

Can we say that video can be a map and link to the spec?

+   *                    { 'minAspectRatio': 1.333, 'maxAspectRatio': 1.334 },
+   *                 'optional': 
+   *                    [{ 'minFrameRate': 60 },
+   *                     { 'maxWidth': 640 }]
+   *     }).then((stream) {
    *       var video = new VideoElement()
    *         ..autoplay = true
    *         ..src = Url.createObjectUrl(stream);
    *       document.body.append(video);
    *     });
    *
    * See also:
    * * [MediaStream.supported]
    */
   @DomName('Navigator.webkitGetUserMedia')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @Experimental
-  Future<LocalMediaStream> getUserMedia({bool audio: false,
-      bool video: false}) {
+  Future<LocalMediaStream> getUserMedia({audio: false,
+      video: false}) {
     var completer = new Completer<LocalMediaStream>();
     var options = {
       'audio': audio,
       'video': video
     };
 $if DART2JS
     _ensureGetUserMedia();
-    this._getUserMedia(convertDartToNative_Dictionary(options),
+    this._getUserMedia(convertDartToNative_SerializedScriptValue(options),
       (stream) {
         completer.complete(stream);
       },
       (error) {
         completer.completeError(error);
       });
 $else
     this._getUserMedia(options,
       (stream) {
         completer.complete(stream);
@@ skipping 14 matching lines @@
     }
   }
 
   @JSName('getUserMedia')
   void _getUserMedia(options, _NavigatorUserMediaSuccessCallback success,
       _NavigatorUserMediaErrorCallback error) native;
 $endif
 
 $!MEMBERS
 }