Convert the code in src/core/worker.js to use ES6 classes

This removes one instance of `// eslint-disable-next-line no-shadow`, which our old pseudo-classes necessitated.

*Please note:* I'm purposely not doing any `var` to `let`/`const` conversion here, since it's generally better to (if possible) do that automatically on e.g. a directory basis instead.

src/core/worker.js

@@ -37,39 +37,34 @@ import { MessageHandler } from "../shared/message_handler.js";
 import { PDFWorkerStream } from "./worker_stream.js";
 import { XRefParseException } from "./core_utils.js";
 
-var WorkerTask = (function WorkerTaskClosure() {
+class WorkerTask {
-  // eslint-disable-next-line no-shadow
+  constructor(name) {
-  function WorkerTask(name) {
     this.name = name;
     this.terminated = false;
     this._capability = createPromiseCapability();
   }
 
-  WorkerTask.prototype = {
   get finished() {
     return this._capability.promise;
-    },
+  }
 
   finish() {
     this._capability.resolve();
-    },
+  }
 
   terminate() {
     this.terminated = true;
-    },
+  }
 
   ensureNotTerminated() {
     if (this.terminated) {
       throw new Error("Worker task was terminated");
     }
-    },
+  }
-  };
+}
 
-  return WorkerTask;
+class WorkerMessageHandler {
-})();
+  static setup(handler, port) {
-
-var WorkerMessageHandler = {
-  setup(handler, port) {
     var testMessageProcessed = false;
     handler.on("test", function wphSetupTest(data) {
       if (testMessageProcessed) {
@@ -96,8 +91,9 @@ var WorkerMessageHandler = {
     handler.on("GetDocRequest", function wphSetupDoc(data) {
       return WorkerMessageHandler.createDocumentHandler(data, port);
     });
-  },
+  }
-  createDocumentHandler(docParams, port) {
+
+  static createDocumentHandler(docParams, port) {
     // This context is actually holds references on pdfManager and handler,
     // until the latter is destroyed.
     var pdfManager;
@@ -662,13 +658,14 @@ var WorkerMessageHandler = {
       docParams = null; // we don't need docParams anymore -- saving memory.
     });
     return workerHandlerName;
-  },
+  }
-  initializeFromPort(port) {
+
+  static initializeFromPort(port) {
     var handler = new MessageHandler("worker", "main", port);
     WorkerMessageHandler.setup(handler, port);
     handler.send("ready", null);
-  },
+  }
-};
+}
 
 function isMessagePort(maybePort) {
   return (