Upgrade Sky's use of the JS bindings

sky/framework/xmlhttprequest.sky

 <import src="/gen/mojo/public/sky/core.sky" as="core" />
 <import src="/gen/mojo/public/sky/unicode.sky" as="unicode" />
 <import src="/gen/mojo/services/network/public/interfaces/network_service.mojom.sky" as="net" />
 <import src="/gen/mojo/services/network/public/interfaces/url_loader.mojom.sky" as="loader" />
 <import src="shell.sky" as="shell" />
 <script>
 // XHR keeps itself alive.
 var outstandingRequests = new Set();
 
 const kPrivate = Symbol("XMLHttpRequestPrivate");
@@ skipping 83 matching lines @@
       // 'body' is actually an array of body segments.
       priv.request.body = [dataPipe.consumerHandle];
     }
 
     var requestHeaders = [];
     priv.headers.forEach(function(value, key) {
       requestHeaders.push(key + ': ' + value);
     });
     priv.request.headers = requestHeaders;
 
-    // FIXME: Factor this into the JS bindings.
-    var pipe = new core.createMessagePipe();
-    priv.networkService.createURLLoader(pipe.handle1);
-    priv.loader = shell.wrapHandle(pipe.handle0, loader.URLLoader);
+    priv.networkService.createURLLoader(function(urlLoaderProxy) {
+        priv.loader = urlLoaderProxy;
+    });
 
     var self = this;
     outstandingRequests.add(this);
     priv.loader.start(priv.request).then(function(result) {
       self.status = result.response.status_code;
       self.statusText = result.response.status_line;
       if (result.response.error)
         throw new Error(result.response.error.description);
       return core.drainData(result.response.body).then(function(result) {
         outstandingRequests.delete(self);
         priv.responseArrayBuffer = result.buffer;
         // Use a setTimeout to avoid exceptions in onload tripping onerror.
         window.setTimeout(function() {
           self.onload();
         });
       });
     }).catch(function(error) {
       outstandingRequests.delete(self);
       // Technically this should throw a ProgressEvent.
       self.onerror(error);
     });
   }
 }
 
 module.exports = XMLHttpRequest;
 </script>