Re-use PromiseResolver helper in print preview.

ui/webui/resources/js/promise_resolver.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview PromiseResolver is a helper class that allows creating a
  * Promise that will be fulfilled (resolved or rejected) some time later.
  *
  * Example:
  *  var resolver = new PromiseResolver();
  *  resolver.promise.then(function(result) {
  *    console.log('resolved with', result);
  *  });
  *  ...
  *  ...
  *  resolver.resolve({hello: 'world'});
  */
 
-/** @constructor */
+/**
+ * @constructor
+ * @template T
+ */
 function PromiseResolver() {
-  /** @type {!Function} */
+  /** @type {function(T): void} */
   this.resolve;
 
-  /** @type {!Function} */
+  /** @type {function(*=): void} */
   this.reject;

[Dan Beam, 2016/03/04 19:29:26]

maybe you already mentioned this (as I remember vaguely bringing this up
before), but why can't we just do this inside the promise callback? i.e.

/**
 * @constructor
 * @template T
 */
function PromiseResolver() {
  this.promise = new Promise(function(resolve, reject) {
    /** @type {function(T): void} */
    this.resolve = resolve;

    /** @type {function(*=): void} */
    this.reject = reject;
  }.bind(this));
}

(even if we need an @this {PromiseResolver} for the compiler to better
understand the scope in the callback)

[dpapad, 2016/03/04 20:01:28]

Thanks for asking, this reminded me that I forgot to add the @struct annotation.
The reason for not declaring this.resolve and this.reject in the callback can be
seen by trying the following example (use "Advanced" compilation to get all the
checks, then click compile, then see the "warnings" tab). 

https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClo...

If you remove the @struct it compiles just fine, but I would like to keep the
@struct annotation because it is very useful, since it catches silly errors
(this.myTyppo = 'foo', instead of this.myTypo= 'foo'). ES6 Classes will be
@struct by default, see
https://github.com/google/closure-compiler/wiki/ECMAScript6. There has been talk
about making @constructor imply @struct, but that work has not been completed
AFAIK.

 
-  /** @type {!Promise} */
+  /** @type {!Promise<T>} */
   this.promise = new Promise(function(resolve, reject) {
     this.resolve = resolve;
     this.reject = reject;
   }.bind(this));
 }