Re-use PromiseResolver helper in print preview.

ui/webui/resources/js/promise_resolver.js

Index: ui/webui/resources/js/promise_resolver.js
diff --git a/ui/webui/resources/js/promise_resolver.js b/ui/webui/resources/js/promise_resolver.js
index e4fedaa5d8c27cff278ccbd3804a7693aecce134..93099909fd84520356a30650d1ab8b47941aa4d1 100644
--- a/ui/webui/resources/js/promise_resolver.js
+++ b/ui/webui/resources/js/promise_resolver.js
@@ -16,15 +16,18 @@
  *  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;