add ast return transform option

lib/6to5/file.js

@@ -41,7 +41,8 @@ File.normaliseOptions = function (opts) {
     filename:     "unknown",
     modules:      "common",
     runtime:      false,
-    code:         true
+    code:         true,
+    ast:          true
   });
 
   // normalise windows path separators to unix
@@ -186,15 +187,18 @@ File.prototype.generate = function () {
   var opts = this.opts;
   var ast  = this.ast;
 
-  if (!opts.code) {
-    return {
-      code: "",
-      map:  null,
-      ast:  ast
-    };
-  }
+  var result = {
+    code: "",
+    map: null,
+    ast: null
+  };
 
-  var result = generate(ast, opts, this.code);
+  if (opts.ast) result.ast = ast;
+  if (!opts.code) return result;
+
+  var _result = generate(ast, opts, this.code);
+  result.code = _result.code;
+  result.map  = _result.map;
 
   if (this.shebang) {
     // add back shebang
@@ -205,8 +209,6 @@ File.prototype.generate = function () {
     result.code += "\n" + util.sourceMapToComment(result.map);
   }
 
-  result.ast = result;
-
   return result;
 };
 

lib/6to5/register.js

@@ -89,7 +89,8 @@ var loader = function (m, filename) {
     whitelist: whitelist,
     blacklist: blacklist,
     sourceMap: true,
-    modules:   "commonInterop"
+    modules:   "commonInterop",
+    ast:       false
   }, transformOpts));
 
   if (cache) {