abstract away astRun from transformer

lib/6to5/transformation/transformer.js

@@ -23,19 +23,22 @@ Transformer.normalise = function (transformer) {
   return transformer;
 };
 
+Transformer.prototype.astRun = function (file, key) {
+  var transformer = this.transformer;
+  var ast         = file.ast;
+
+  if (transformer.ast && transformer.ast[key]) {
+    transformer.ast[key](ast, file);
+  }
+};
+
 Transformer.prototype.transform = function (file) {
   if (!this.canRun(file)) return;
 
   var transformer = this.transformer;
   var ast         = file.ast;
 
-  var astRun = function (key) {
-    if (transformer.ast && transformer.ast[key]) {
-      transformer.ast[key](ast, file);
-    }
-  };
-
-  astRun("enter");
+  this.astRun(file, "enter");
 
   var build = function (exit) {
     return function (node, parent, scope) {
@@ -64,7 +67,7 @@ Transformer.prototype.transform = function (file) {
     exit:  build(true)
   });
 
-  astRun("exit");
+  this.astRun(file, "exit");
 };
 
 Transformer.prototype.canRun = function (file) {