Adding ability to specify request headers and mime type for Future-based XHRs.

tools/dom/templates/html/impl/impl_XMLHttpRequest.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;
 
 /**
  * A utility for retrieving data from a URL.
  *
  * HttpRequest can be used to obtain data from http, ftp, and file
@@ skipping 60 matching lines @@
    * * If `Access-Control-Expose-Headers` has not been set to true, only a subset of all the response headers will be returned when calling [getAllRequestHeaders].
    *
    * Note that requests for file:// URIs are only supported by Chrome extensions
    * with appropriate permissions in their manifest. Requests to file:// URIs
    * will also never fail- the Future will always complete successfully, even
    * when the file cannot be found.
    *
    * See also: [authorization headers](http://en.wikipedia.org/wiki/Basic_access_authentication).
    */
   static Future<HttpRequest> request(String url,
-      {String method, bool withCredentials, String responseType, sendData,
+      {String method, bool withCredentials, String responseType,
+      String mimeType, Map<String, String> requestHeaders, sendData,
       void onProgress(ProgressEvent e)}) {
     var completer = new Completer<HttpRequest>();
 
     var xhr = new HttpRequest();
     if (method == null) {
       method = 'GET';
     }
     xhr.open(method, url, async: true);
 
     if (withCredentials != null) {
       xhr.withCredentials = withCredentials;
     }
 
     if (responseType != null) {
       xhr.responseType = responseType;
     }
 
+    if (mimeType != null) {
+      xhr.overrideMimeType(mimeType);
+    }
+
+    if (requestHeaders != null) {
+      requestHeaders.forEach((header, value) {
+        xhr.setRequestHeader(header, value);
+      });
+    }
+
     if (onProgress != null) {
       xhr.onProgress.listen(onProgress);
     }
 
     xhr.onLoad.listen((e) {
       // Note: file:// URIs have status of 0.
       if ((xhr.status >= 200 && xhr.status < 300) ||
           xhr.status == 0 || xhr.status == 304) {
         completer.complete(xhr);
       } else {
@@ skipping 49 matching lines @@
 $if DART2JS
     var xhr = new HttpRequest();
     return JS('bool', '("onloadend" in #)', xhr);
 $else
     return true;
 $endif
   }
 
 $!MEMBERS
 }