require both flags

Author: ieviev

src/fable-library-ts/RegExp.ts

@@ -18,7 +18,14 @@ export function create(pattern: string, options = 0) {
   flags += options & 1 ? "i" : ""; // 0x0001 RegexOptions.IgnoreCase
   flags += options & 2 ? "m" : "";
   flags += options & 16 ? "s" : "";
-  flags += options & 1024 ? "l" : "";
+  if (options & 1024) {
+    if (options & 256) {
+      flags += "l";
+    }
+    else {
+      throw new Error("RegexOptions.NonBacktracking is only allowed together with ECMAScript");
+    }
+  }
   return new RegExp(pattern, flags);
 }
 // From http://stackoverflow.com/questions/3446170/escape-string-for-use-in-javascript-regex