refactor(http): remove a now useless workaround

dartbug.com/13051 has been fixed.
see https://code.google.com/p/dart/issues/detail?id=13051 for details.

Closes #523

lib/core_dom/http.dart

@@ -22,21 +22,11 @@ class HttpBackend {
   async.Future request(String url,
       {String method, bool withCredentials, String responseType,
       String mimeType, Map<String, String> requestHeaders, sendData,
-      void onProgress(dom.ProgressEvent e)}) {
-    // Complete inside a then to work-around dartbug.com/13051
-    var c = new async.Completer();
-
-    dom.HttpRequest.request(url,
-        method: method,
-        withCredentials: withCredentials,
-        responseType: responseType,
-        mimeType: mimeType,
-        requestHeaders: requestHeaders,
-        sendData: sendData,
-        onProgress: onProgress).then((x) => c.complete(x),
-        onError: (e, stackTrace) => c.completeError(e, stackTrace));
-    return c.future;
-  }
+      void onProgress(dom.ProgressEvent e)}) =>
+      dom.HttpRequest.request(url, method: method,
+        withCredentials: withCredentials, responseType: responseType,
+        mimeType: mimeType, requestHeaders: requestHeaders,
+        sendData: sendData, onProgress: onProgress);
 }
 
 @NgInjectableService()