Require ECMA-402 support from ECMA-262 implementations

Fixes whatwg#1976.

source

@@ -2981,6 +2981,9 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <p>Some parts of the language described by this specification only support JavaScript as the
     underlying scripting language. <ref spec=JAVASCRIPT></p>
 
+    <p>Users agents that support JavaScript must also implement the <cite>ECMAScript
+    Internationalization API Specification</cite>. <ref spec=JSINTL></p>
+
     <p class="note">The term "JavaScript" is used to refer to ECMA-262, rather than the official
     term ECMAScript, since the term JavaScript is more widely known. Similarly, the <span>MIME
     type</span> used to refer to JavaScript in this specification is <code
@@ -118673,6 +118676,9 @@ INSERT INTERFACES HERE
    <dt id="refsJPEG">[JPEG]</dt>
    <dd><cite><a href="https://www.w3.org/Graphics/JPEG/jfif3.pdf">JPEG File Interchange Format</a></cite>, E. Hamilton.</dd>
 
+   <dt id="refsJSINTL">[JSINTL]</dt>
+   <dd><cite><a href="https://tc39.github.io/ecma402/">ECMAScript Internationalization API Specification</a></cite>. Ecma International.</dd>
+
    <dt id="refsJSON">[JSON]</dt>
    <dd><cite><a href="https://tools.ietf.org/html/rfc7159">The JavaScript Object Notation (JSON) Data Interchange Format</a></cite>, T. Bray. IETF.</dd>