parse asnyc closures

Author: m4rch3n1ng

grammar.js

@@ -1293,6 +1293,7 @@ module.exports = grammar({
 
     closure_expression: $ => prec(PREC.closure, seq(
       optional('static'),
+      optional('async'),
       optional('move'),
       field('parameters', $.closure_parameters),
       choice(

test/corpus/async.txt

@@ -87,6 +87,37 @@ async move {}
     (async_block
       (block))))
 
+================================================================================
+Async closure
+================================================================================
+
+let _ = async || ();
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (let_declaration
+    (closure_expression
+      (closure_parameters)
+      (unit_expression))))
+
+================================================================================
+Async move async move closure
+================================================================================
+
+let a = async move || async move {};
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (let_declaration
+    (identifier)
+    (closure_expression
+      (closure_parameters)
+      (async_block
+        (block)))))
+
+
 ================================================================================
 Try Block
 ================================================================================