Use generated function instead of eval to get async func ctor

lib/ejs.js

@@ -551,7 +551,7 @@ Template.prototype = {
     var prepended = '';
     var appended = '';
     var escapeFn = opts.escapeFunction;
-    var constructor;
+    var asyncCtor;
 
     if (!this.source) {
       this.generateSource();
@@ -598,9 +598,10 @@ Template.prototype = {
 
     try {
       if (opts.async) {
-        // Have to use eval for this, since in envs without support, it breaks in parsing
+        // Have to use generated function for this, since in envs without support,
+        // it breaks in parsing
         try {
-          constructor = eval('(async function(){})').constructor;
+          asyncCtor = (new Function('return (async function(){}).constructor;'))();
         }
         catch(e) {
           if (e instanceof SyntaxError) {
@@ -612,9 +613,9 @@ Template.prototype = {
         }
       }
       else {
-        constructor = Function;
+        asyncCtor = Function;
       }
-      fn = new constructor(opts.localsName + ', escapeFn, include, rethrow', src);
+      fn = new asyncCtor(opts.localsName + ', escapeFn, include, rethrow', src);
     }
     catch(e) {
       // istanbul ignore else