(dart) highlight built-in nullable types

src/languages/dart.js

@@ -72,17 +72,28 @@ export default function(hljs) {
     hljs.C_NUMBER_MODE, STRING
   ];
 
+  var BUILT_IN_TYPES =
+    // dart:core
+    'Comparable DateTime Duration Function Iterable Iterator List Map Match Object Pattern RegExp Set Stopwatch ' +
+    'String StringBuffer StringSink Symbol Type Uri bool double int num ' +
+    // dart:html
+    'Element ElementList';
+
   var KEYWORDS = {
     keyword: 'abstract as assert async await break case catch class const continue covariant default deferred do ' +
       'dynamic else enum export extends extension external factory false final finally for Function get hide if ' +
       'implements import in inferface is late library mixin new null on operator part required rethrow return set ' +
       'show static super switch sync this throw true try typedef var void while with yield',
     built_in:
+      // non-nullable built-in types
+      BUILT_IN_TYPES + ' ' +
+      // nullable built-in types
+      BUILT_IN_TYPES.split(' ').map((e) => { e + '?' }).join(' ') + ' ' +
       // dart:core
-      'Comparable DateTime Duration Function Iterable Iterator List Map Match Never Null Object Pattern RegExp ' +
-      'Set Stopwatch String StringBuffer StringSink Symbol Type Uri bool double dynamic int num print ' +
+      'Never Null dynamic print ' +
       // dart:html
-      'Element ElementList document querySelector querySelectorAll window'
+      'document querySelector querySelectorAll window',
+    $pattern: /[A-Za-z][A-Za-z0-9_]+\??/
   };
 
   return {