Update flatten helper without recursion (jashkenas#5454)

docs/v2/annotated-source/coffeescript.html

@@ -824,7 +824,7 @@ <h1>coffeescript.coffee</h1>
     <span class="hljs-string">&quot;<span class="hljs-subst">#{err.toString()}</span>\n<span class="hljs-subst">#{frames.join <span class="hljs-string">&#x27;\n&#x27;</span>}</span>\n&quot;</span>
 <span class="hljs-function">
 <span class="hljs-title">checkShebangLine</span> = <span class="hljs-params">(file, input)</span> -&gt;</span>
-  firstLine = input.split(<span class="hljs-regexp">/$/m</span>)[<span class="hljs-number">0</span>]
+  firstLine = input.split(<span class="hljs-regexp">/$/m</span>, <span class="hljs-number">1</span>)[<span class="hljs-number">0</span>]
   rest = firstLine?.match(<span class="hljs-regexp">/^#!\s*([^\s]+\s*)(.*)/</span>)
   args = rest?[<span class="hljs-number">2</span>]?.split(<span class="hljs-regexp">/\s/</span>).filter (s) -&gt; s <span class="hljs-keyword">isnt</span> <span class="hljs-string">&#x27;&#x27;</span>
   <span class="hljs-keyword">if</span> args?.length &gt; <span class="hljs-number">1</span>

docs/v2/annotated-source/helpers.html

@@ -275,13 +275,7 @@ <h1>helpers.coffee</h1>
             </div>
 
             <div class="content"><div class='highlight'><pre><span class="hljs-built_in">exports</span>.flatten = flatten = <span class="hljs-function"><span class="hljs-params">(array)</span> -&gt;</span>
-  flattened = []
-  <span class="hljs-keyword">for</span> element <span class="hljs-keyword">in</span> array
-    <span class="hljs-keyword">if</span> <span class="hljs-string">&#x27;[object Array]&#x27;</span> <span class="hljs-keyword">is</span> Object::toString.call element
-      flattened = flattened.concat flatten element
-    <span class="hljs-keyword">else</span>
-      flattened.push element
-  flattened</pre></div></div>
+  array.flat(<span class="hljs-literal">Infinity</span>)</pre></div></div>
 
         </li>
 

src/helpers.coffee

@@ -47,13 +47,7 @@ extend = exports.extend = (object, properties) ->
 # Return a flattened version of an array.
 # Handy for getting a list of `children` from the nodes.
 exports.flatten = flatten = (array) ->
-  flattened = []
-  for element in array
-    if '[object Array]' is Object::toString.call element
-      flattened = flattened.concat flatten element
-    else
-      flattened.push element
-  flattened
+  array.flat(Infinity)
 
 # Delete a key from an object, returning the value. Useful when a node is
 # looking for a particular method in an options hash.