[javascript mode] Handle TypeScript 'abstract' keyword

Closes codemirror#5333

Author: marijnh

mode/javascript/javascript.js

@@ -356,6 +356,9 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       } else if (isTS && value == "namespace") {
         cx.marked = "keyword"
         return cont(pushlex("form"), expression, block, poplex)
+      } else if (isTS && value == "abstract") {
+        cx.marked = "keyword"
+        return cont(statement)
       } else {
         return cont(pushlex("stat"), maybelabel);
       }

mode/javascript/test.js

@@ -408,6 +408,9 @@
      "  [keyword private] [property bar]();",
      "}")
 
+  TS("abstract class",
+     "[keyword export] [keyword abstract] [keyword class] [def Foo] {}")
+
   var jsonld_mode = CodeMirror.getMode(
     {indentUnit: 2},
     {name: "javascript", jsonld: true}