Replace classes.subclass with native classes

See #442.

lib/grammar-error.js

@@ -1,18 +1,16 @@
 "use strict";
 
-let classes = require("./utils/classes");
-
 /* Thrown when the grammar contains an error. */
-function GrammarError(message, location) {
-  this.name = "GrammarError";
-  this.message = message;
-  this.location = location;
+class GrammarError {
+  constructor(message, location) {
+    this.name = "GrammarError";
+    this.message = message;
+    this.location = location;
 
-  if (typeof Error.captureStackTrace === "function") {
-    Error.captureStackTrace(this, GrammarError);
+    if (typeof Error.captureStackTrace === "function") {
+      Error.captureStackTrace(this, GrammarError);
+    }
   }
 }
 
-classes.subclass(GrammarError, Error);
-
 module.exports = GrammarError;

lib/utils/classes.js

@@ -1,12 +0,0 @@
-"use strict";
-
-/* Class utilities */
-let classes = {
-  subclass: function(child, parent) {
-    function ctor() { this.constructor = child; }
-    ctor.prototype = parent.prototype;
-    child.prototype = new ctor();
-  }
-};
-
-module.exports = classes;

package.json

@@ -37,7 +37,6 @@
     "lib/parser.js",
     "lib/peg.js",
     "lib/utils/arrays.js",
-    "lib/utils/classes.js",
     "lib/utils/objects.js",
     "package.json"
   ],