Extern xhr error property

Make it possible to access the response when a request fail

#317

externs/errors.js

@@ -0,0 +1,26 @@
+/**
+ * @license
+ * Copyright 2016 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @fileoverview Externs for additional Error properties.
+ *
+ * @externs
+ */
+
+
+/** @type {XMLHttpRequest} */
+Error.xhr;

spec/ajax_request_spec.js

@@ -117,5 +117,13 @@ describe('AjaxRequest', function() {
       done();
     });
   }
-});
 
+  it('instance should be available as xhr property on error', function(done) {
+    var ajaxRequest = new shaka.util.AjaxRequest('/');
+    ajaxRequest.send().catch(function(error) {
+      expect(error.xhr).toEqual(jasmine.any(XMLHttpRequest));
+      done();
+    });
+    ajaxRequest.abort();
+  });
+});