Fix incorrect variable name on two places

src/checks.js

@@ -66,7 +66,7 @@ PEG.compiler.checks = [
       grammar:
         function(node, appliedRules) {
           for (var name in node.rules) {
-            check(ast.rules[name], appliedRules);
+            check(node.rules[name], appliedRules);
           }
         },
 

src/passes.js

@@ -32,7 +32,7 @@ PEG.compiler.passes = [
         grammar:
           function(node, from, to) {
             for (var name in node.rules) {
-              replace(ast.rules[name], from, to);
+              replace(node.rules[name], from, to);
             }
           },