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

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 0e2dbb5a83a583dff1dfcf0d71270bf919acb17f..ae35cf91d8a063568ed35e5fa83f751a4b33d7bd 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -10573,7 +10573,8 @@ class HttpRequest extends EventTarget native "XMLHttpRequest" {
    * 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>();
 
@@ -10591,6 +10592,16 @@ class HttpRequest extends EventTarget native "XMLHttpRequest" {
       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);
     }