Add cancel method to fetch mixin.

fetch-mixin.js

@@ -5,6 +5,10 @@ export const fetchMixin = function(superClass) {
       this.__abortControllers = new Map();
     }
 
+    cancel(method, url) {
+      return this.__cancelRunningRequest(method, url);
+    }
+
     get(url, overwrite = true) {
       return this.do('GET', url, null, overwrite);
     }