Move JS Modules to the "allowed features" of the WebUI ES styleguide.

Also add a new "Dynamic import" entry, in the "To Be Discussed" section,
to ensure that we don't accidentally allow dynamic imports before examining
any complications (for example ESLint breakage) further.

Bug: 915053
Change-Id: I0f166cabcedfd4cc8cc445ad177e5344d7ac8b71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733576
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691394}

styleguide/web/es.md

@@ -563,6 +563,31 @@ const [one, ...rest] = [1, 2, 3];
 
 ---
 
+### Modules
+
+Support for exporting/importing values from/to modules without global
+namespace pollution.
+
+**Usage Example:**
+
+```js
+// lib/rect.js
+export function getArea() {...};
+export {width, height, unimportant};
+
+// app.js
+import {getArea, width, height} from './lib/rect.js';
+```
+
+**Documentation:** [link](https://developers.google.com/web/fundamentals/primers/modules)
+
+**Discussion Notes / Link to Thread:**
+Dynamic Import [link](https://v8.dev/features/dynamic-import) are not allowed
+yet, see separate entry in the [Features To Be Discussed](##es2015-support-in-chromium-features-to-be-discussed)
+section.
+
+---
+
 ## Banned Features
 
 The following features are banned for Chromium development.
@@ -730,12 +755,10 @@ result === 'yy' && re.lastIndex === 5;  // true
 
 **Discussion Notes / Link to Thread:**
 
----
+### Dynamic Import
 
-### Modules
-
-Support for exporting/importing values from/to modules without global
-namespace pollution.
+Dynamic import() introduces a new function-like form of import that returns a
+promise for the module namespace object of the requested module.
 
 **Usage Example:**
 
@@ -745,10 +768,14 @@ export function getArea() {...};
 export {width, height, unimportant};
 
 // app.js
-import {getArea, width, height} from './lib/rect.js';
+if (calculateArea) {
+  import('./lib/rect.js').then(rect => {
+    rect.getArea(...);
+  });
+}
 ```
 
-**Documentation:** [link](https://developers.google.com/web/fundamentals/primers/modules)
+**Documentation:** [link](https://v8.dev/features/dynamic-import)
 
 **Discussion Notes / Link to Thread:**