workers: basic event loop (#3828)

* establish basic event loop for workers
* make "self.close()" inside worker
* remove "runWorkerMessageLoop() - instead manually call global function 
  in Rust when message arrives. This is done in preparation for structured clone
* refactor "WorkerChannel" and use distinct structs for internal 
  and external channels;  "WorkerChannelsInternal" and "WorkerHandle"
* move "State.worker_channels_internal" to "Worker.internal_channels"
* add "WorkerEvent" enum for child->host communication; 
  currently "Message(Buf)" and  "Error(ErrBox)" variants are supported
* add tests for nested workers
* add tests for worker throwing error on startup

cli/js/workers_test.ts

@@ -0,0 +1,84 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { test, assert, assertEquals } from "./test_util.ts";
+
+export interface ResolvableMethods<T> {
+  resolve: (value?: T | PromiseLike<T>) => void;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  reject: (reason?: any) => void;
+}
+
+export type Resolvable<T> = Promise<T> & ResolvableMethods<T>;
+
+export function createResolvable<T>(): Resolvable<T> {
+  let methods: ResolvableMethods<T>;
+  const promise = new Promise<T>((resolve, reject): void => {
+    methods = { resolve, reject };
+  });
+  // TypeScript doesn't know that the Promise callback occurs synchronously
+  // therefore use of not null assertion (`!`)
+  return Object.assign(promise, methods!) as Resolvable<T>;
+}
+
+test(async function workersBasic(): Promise<void> {
+  const promise = createResolvable();
+  const jsWorker = new Worker("../tests/subdir/test_worker.js", {
+    type: "module",
+    name: "jsWorker"
+  });
+  const tsWorker = new Worker("../tests/subdir/test_worker.ts", {
+    type: "module",
+    name: "tsWorker"
+  });
+
+  tsWorker.onmessage = (e): void => {
+    assertEquals(e.data, "Hello World");
+    promise.resolve();
+  };
+
+  jsWorker.onmessage = (e): void => {
+    assertEquals(e.data, "Hello World");
+    tsWorker.postMessage("Hello World");
+  };
+
+  jsWorker.onerror = (e: Event): void => {
+    e.preventDefault();
+    jsWorker.postMessage("Hello World");
+  };
+
+  jsWorker.postMessage("Hello World");
+  await promise;
+});
+
+test(async function nestedWorker(): Promise<void> {
+  const promise = createResolvable();
+
+  const nestedWorker = new Worker("../tests/subdir/nested_worker.js", {
+    type: "module",
+    name: "nested"
+  });
+
+  nestedWorker.onmessage = (e): void => {
+    assert(e.data.type !== "error");
+    promise.resolve();
+  };
+
+  nestedWorker.postMessage("Hello World");
+  await promise;
+});
+
+test(async function workerThrowsWhenExecuting(): Promise<void> {
+  const promise = createResolvable();
+
+  const throwingWorker = new Worker("../tests/subdir/throwing_worker.js", {
+    type: "module"
+  });
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  throwingWorker.onerror = (e: any): void => {
+    e.preventDefault();
+    assertEquals(e.message, "Uncaught Error: Thrown error");
+    promise.resolve();
+  };
+
+  await promise;
+});