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diff --git a/cli/js/util.ts b/cli/js/util.ts
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+++ b/cli/js/util.ts
@@ -0,0 +1,225 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+import { TypedArray } from "./types.ts";
+import { window } from "./window.ts";
+
+let logDebug = false;
+let logSource = "JS";
+
+// @internal
+export function setLogDebug(debug: boolean, source?: string): void {
+  logDebug = debug;
+  if (source) {
+    logSource = source;
+  }
+}
+
+/** Debug logging for deno.
+ * Enable with the `--log-debug` or `-D` command line flag.
+ * @internal
+ */
+export function log(...args: unknown[]): void {
+  if (logDebug) {
+    // if we destructure `console` off `window` too early, we don't bind to
+    // the right console, therefore we don't log anything out.
+    window.console.log(`DEBUG ${logSource} -`, ...args);
+  }
+}
+
+// @internal
+export function assert(cond: boolean, msg = "assert"): void {
+  if (!cond) {
+    throw Error(msg);
+  }
+}
+
+// @internal
+export function typedArrayToArrayBuffer(ta: TypedArray): ArrayBuffer {
+  const ab = ta.buffer.slice(ta.byteOffset, ta.byteOffset + ta.byteLength);
+  return ab as ArrayBuffer;
+}
+
+// @internal
+export function arrayToStr(ui8: Uint8Array): string {
+  return String.fromCharCode(...ui8);
+}
+
+/** A `Resolvable` is a Promise with the `reject` and `resolve` functions
+ * placed as methods on the promise object itself. It allows you to do:
+ *
+ *       const p = createResolvable<number>();
+ *       // ...
+ *       p.resolve(42);
+ *
+ * It'd be prettier to make `Resolvable` a class that inherits from `Promise`,
+ * rather than an interface. This is possible in ES2016, however typescript
+ * produces broken code when targeting ES5 code.
+ *
+ * At the time of writing, the GitHub issue is closed in favour of a proposed
+ * solution that is awaiting feedback.
+ *
+ * @see https://github.com/Microsoft/TypeScript/issues/15202
+ * @see https://github.com/Microsoft/TypeScript/issues/15397
+ * @internal
+ */
+
+export interface ResolvableMethods<T> {
+  resolve: (value?: T | PromiseLike<T>) => void;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  reject: (reason?: any) => void;
+}
+
+// @internal
+export type Resolvable<T> = Promise<T> & ResolvableMethods<T>;
+
+// @internal
+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>;
+}
+
+// @internal
+export function notImplemented(): never {
+  throw new Error("Not implemented");
+}
+
+// @internal
+export function unreachable(): never {
+  throw new Error("Code not reachable");
+}
+
+// @internal
+export function hexdump(u8: Uint8Array): string {
+  return Array.prototype.map
+    .call(
+      u8,
+      (x: number): string => {
+        return ("00" + x.toString(16)).slice(-2);
+      }
+    )
+    .join(" ");
+}
+
+// @internal
+export function containsOnlyASCII(str: string): boolean {
+  if (typeof str !== "string") {
+    return false;
+  }
+  return /^[\x00-\x7F]*$/.test(str);
+}
+
+const TypedArrayConstructor = Object.getPrototypeOf(Uint8Array);
+export function isTypedArray(x: unknown): x is TypedArray {
+  return x instanceof TypedArrayConstructor;
+}
+
+// Returns whether o is an object, not null, and not a function.
+// @internal
+export function isObject(o: unknown): o is object {
+  return o != null && typeof o === "object";
+}
+
+// Returns whether o is iterable.
+export function isIterable<T, P extends keyof T, K extends T[P]>(
+  o: T
+): o is T & Iterable<[P, K]> {
+  // checks for null and undefined
+  if (o == null) {
+    return false;
+  }
+  return (
+    typeof ((o as unknown) as Iterable<[P, K]>)[Symbol.iterator] === "function"
+  );
+}
+
+// @internal
+export function requiredArguments(
+  name: string,
+  length: number,
+  required: number
+): void {
+  if (length < required) {
+    const errMsg = `${name} requires at least ${required} argument${
+      required === 1 ? "" : "s"
+    }, but only ${length} present`;
+    throw new TypeError(errMsg);
+  }
+}
+
+// @internal
+export function immutableDefine(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  o: any,
+  p: string | number | symbol,
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  value: any
+): void {
+  Object.defineProperty(o, p, {
+    value,
+    configurable: false,
+    writable: false
+  });
+}
+
+// Returns values from a WeakMap to emulate private properties in JavaScript
+export function getPrivateValue<
+  K extends object,
+  V extends object,
+  W extends keyof V
+>(instance: K, weakMap: WeakMap<K, V>, key: W): V[W] {
+  if (weakMap.has(instance)) {
+    return weakMap.get(instance)![key];
+  }
+  throw new TypeError("Illegal invocation");
+}
+
+/**
+ * Determines whether an object has a property with the specified name.
+ * Avoid calling prototype builtin `hasOwnProperty` for two reasons:
+ *
+ * 1. `hasOwnProperty` is defined on the object as something else:
+ *
+ *      const options = {
+ *        ending: 'utf8',
+ *        hasOwnProperty: 'foo'
+ *      };
+ *      options.hasOwnProperty('ending') // throws a TypeError
+ *
+ * 2. The object doesn't inherit from `Object.prototype`:
+ *
+ *       const options = Object.create(null);
+ *       options.ending = 'utf8';
+ *       options.hasOwnProperty('ending'); // throws a TypeError
+ *
+ * @param obj A Object.
+ * @param v A property name.
+ * @see https://eslint.org/docs/rules/no-prototype-builtins
+ * @internal
+ */
+export function hasOwnProperty<T>(obj: T, v: PropertyKey): boolean {
+  if (obj == null) {
+    return false;
+  }
+  return Object.prototype.hasOwnProperty.call(obj, v);
+}
+
+/**
+ * Split a number into two parts: lower 32 bit and higher 32 bit
+ * (as if the number is represented as uint64.)
+ *
+ * @param n Number to split.
+ * @internal
+ */
+export function splitNumberToParts(n: number): number[] {
+  // JS bitwise operators (OR, SHIFT) operate as if number is uint32.
+  const lower = n | 0;
+  // This is also faster than Math.floor(n / 0x100000000) in V8.
+  const higher = (n - lower) / 0x100000000;
+  return [lower, higher];
+}