simplify amd module formatter

lib/6to5/modules/amd.js

@@ -18,22 +18,18 @@ AMDFormatter.prototype.transform = function (ast) {
 
   // build an array of module names
 
-  var names = [];
+  var names = [b.literal("exports")];
   _.each(this.ids, function (id, name) {
     names.push(b.literal(name));
   });
   names = b.arrayExpression(names);
 
-  // add exports
-
-  body.unshift(b.variableDeclaration("var", [
-    b.variableDeclarator(b.identifier("exports"), b.objectExpression([]))
-  ]));
-  body.push(b.returnStatement(b.identifier("exports")));
-
   // build up define container
 
-  var container = b.functionExpression(null, _.values(this.ids), b.blockStatement(body));
+  var params = _.values(this.ids);
+  params.unshift(b.identifier("exports"));
+
+  var container = b.functionExpression(null, params, b.blockStatement(body));
   var call = b.callExpression(b.identifier("define"), [names, container]);
 
   program.body = [b.expressionStatement(call)];