Don't GC message pipes that have an in-flight request

sky/framework/xmlhttprequest.sky

 <import src="/mojo/public/sky/core.sky" as="core" />
 <import src="/mojo/public/sky/unicode.sky" as="unicode" />
 <import src="/mojo/services/public/interfaces/network/network_service.mojom.sky" as="net" />
 <import src="/mojo/services/public/interfaces/network/url_loader.mojom.sky" as="loader" />
 <import src="shell.sky" as="shell" />
 <script>
 function XMLHttpRequest() {
   this.networkService_ = shell.connectToService(
     "mojo://network_service/", net.NetworkService);
   this.request_ = null;
@@ skipping 24 matching lines @@
   });
   this.request_.headers = requestHeaders;
 
   // FIXME: Factor this into the JS bindings.
   var pipe = new core.createMessagePipe();
   this.networkService_.createURLLoader(pipe.handle1);
   this.loader_ = shell.wrapHandle(pipe.handle0, loader.URLLoader);
 
   var self = this;
   this.loader_.start(this.request_).then(function(result) {
-    core.drainData(result.response.body).then(function(result) {
+    return core.drainData(result.response.body).then(function(result) {
       self.responseText = unicode.decodeUtf8String(new Uint8Array(result.buffer));
       self.onload();
-    }).catch(function(error) {
-      self.onerror(error);
     });
   }).catch(function(error) {
     self.onerror(error);
   });
 };
 
 module.exports = XMLHttpRequest;
 </script>