use module syntax for strict module formatters

src/babel/helpers/code-frame.js

@@ -54,7 +54,7 @@ function highlight(text) {
   });
 }
 
-export default function (lines: number, lineNumber: number, colNumber: number, opts?): string {
+export default function (lines: number, lineNumber: number, colNumber: number, opts = {}): string {
   colNumber = Math.max(colNumber, 0);
 
   if (opts.highlightCode && chalk.supportsColor) {

src/babel/transformation/modules/amd-strict.js

@@ -1 +1,3 @@
-module.exports = require("./_strict")(require("./amd"));
+import AMDFormatter from "./amd";
+import buildStrict from "./_strict";
+export default buildStrict(AMDFormatter);

src/babel/transformation/modules/common-strict.js

@@ -1 +1,3 @@
-module.exports = require("./_strict")(require("./common"));
+import CommonFormatter from "./common";
+import buildStrict from "./_strict";
+export default buildStrict(CommonFormatter);

src/babel/transformation/modules/system.js

@@ -42,7 +42,7 @@ var hoistVariablesVisitor = {
         }
 
         if (parent.init === node) {
-          return t.toSequenceExpression(nodes, scope);
+          return nodes;
         }
       }
 

src/babel/transformation/modules/umd-strict.js

@@ -1 +1,3 @@
-module.exports = require("./_strict")(require("./umd"));
+import UMDFormatter from "./umd";
+import buildStrict from "./_strict";
+export default buildStrict(UMDFormatter);

src/babel/transformation/transformer.js

@@ -3,7 +3,7 @@ import isFunction from "lodash/lang/isFunction";
 import traverse from "../traversal";
 import isObject from "lodash/lang/isObject";
 import assign from "lodash/object/assign";
-import acorn from "../../../vendor/acorn";
+import * as acorn from "../../acorn";
 import File from "./file";
 import each from "lodash/collection/each";