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

tests/html/xhr_test.dart

Index: tests/html/xhr_test.dart
diff --git a/tests/html/xhr_test.dart b/tests/html/xhr_test.dart
index 9d44746ff1f942c1c4e97440c112c22c4c81012a..a3d7a852155ce289befaf9ad65d79ce16fcaa05c 100644
--- a/tests/html/xhr_test.dart
+++ b/tests/html/xhr_test.dart
@@ -145,7 +145,9 @@ main() {
 
     test('XHR.request responseType arraybuffer', () {
       if (Platform.supportsTypedData) {
-        HttpRequest.request(url, responseType: 'arraybuffer').then(
+        HttpRequest.request(url, responseType: 'arraybuffer',
+          mimeType: 'application/binary',
+          requestHeaders: {'Content-Type': 'text/xml'}).then(
           expectAsync1((xhr) {
             expect(xhr.status, equals(200));
             var byteBuffer = xhr.response;