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

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

Index: tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate b/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
index 7ae7698d7cf50d2180b7cde41b3edf7c8e7103e6..e130052c8747f4c7cee0b821e0c926f196ec6bd8 100644
--- a/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
+++ b/tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate
@@ -78,7 +78,8 @@ $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    * 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>();
 
@@ -96,6 +97,16 @@ $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
       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);
     }