[javascript mode] Support async and await

Closes #3926
Closes #4026

mode/javascript/javascript.js

@@ -42,7 +42,8 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       "in": operator, "typeof": operator, "instanceof": operator,
       "true": atom, "false": atom, "null": atom, "undefined": atom, "NaN": atom, "Infinity": atom,
       "this": kw("this"), "class": kw("class"), "super": kw("atom"),
-      "yield": C, "export": kw("export"), "import": kw("import"), "extends": C
+      "yield": C, "export": kw("export"), "import": kw("import"), "extends": C,
+      "await": C, "async": kw("async")
     };
 
     // Extend the 'normal' keywords with the TypeScript language extensions
@@ -366,6 +367,7 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
     if (type == "export") return cont(pushlex("stat"), afterExport, poplex);
     if (type == "import") return cont(pushlex("stat"), afterImport, poplex);
     if (type == "module") return cont(pushlex("form"), pattern, pushlex("}"), expect("{"), block, poplex, poplex)
+    if (type == "async") return cont(statement)
     return pass(pushlex("stat"), expression, expect(";"), poplex);
   }
   function expression(type) {