fix(fetch): optimize signals composing logic; (#6582)

lib/cancel/CancelToken.js

@@ -102,6 +102,20 @@ class CancelToken {
     }
   }
 
+  toAbortSignal() {
+    const controller = new AbortController();
+
+    const abort = (err) => {
+      controller.abort(err);
+    };
+
+    this.subscribe(abort);
+
+    controller.signal.unsubscribe = () => this.unsubscribe(abort);
+
+    return controller.signal;
+  }
+
   /**
    * Returns an object that contains a new `CancelToken` and a function that, when called,
    * cancels the `CancelToken`.