[javascript mode] Only allow HTML-style comments at the start of line

Closes codemirror#6305

Author: marijnh

mode/javascript/javascript.js

@@ -103,7 +103,8 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       return ret("meta", "meta");
     } else if (ch == "#" && stream.eatWhile(wordRE)) {
       return ret("variable", "property")
-    } else if (ch == "<" && stream.match("!--") || ch == "-" && stream.match("->")) {
+    } else if ((ch == "<" && stream.match("!--") || ch == "-" && stream.match("->")) &&
+               !/\S/.test(stream.string.slice(0, stream.start))) {
       stream.skipToEnd()
       return ret("comment", "comment")
     } else if (isOperatorChar.test(ch)) {