Bug 1063110: use arrow fn to capture outer scope in closure.

lib/task.js

@@ -598,7 +598,7 @@ export default class Task {
       return await this.abortRun(this.taskState);
     }
     // Validate the schema!
-    let payloadErrors = await validator().then(function (validator) {
+    let payloadErrors = await validator().then((validator) => {
       return validator.check(this.task.payload, payloadSchema);
     });