context simplification with fetch API

@@ -53,25 +53,8 @@ ARCS.Context = function( ctx ) {
                 }
             });
         } else {
-            return new Promise(function(resolve, reject) {
-                var client = new XMLHttpRequest();
-                client.open('GET',fileName,true);
-                client.overrideMimeType("application/json");
-                client.send();
-                
-                client.onload = function() {
-                    if (this.status >= 200 && this.status < 300) {
-                        resolve(JSON.parse(this.responseText));
-                    } else {
-                        console.error(this.statusText);
-                        reject(this.statusText);
-                    }                
-                };
-                client.onerror = function() {
-                    console.error(this.statusText);
-                    reject(this.statusText);
-                };
-            });
+            var client = await fetch(fileName);
+            return client.json();
         }
     };