better addImport with good hoisting etc

lib/6to5/file.js

@@ -10,11 +10,11 @@ var t         = require("./types");
 var _         = require("lodash");
 
 function File(opts) {
-  this.unshiftProgramAtEnd = [];
-  this.opts                = File.normaliseOptions(opts);
-  this.transformers        = this.getTransformers();
-  this.uids                = {};
-  this.ast                 = {};
+  this.dynamicImports = [];
+  this.opts           = File.normaliseOptions(opts);
+  this.transformers   = this.getTransformers();
+  this.uids           = {};
+  this.ast            = {};
 }
 
 File.declarations = [
@@ -157,7 +157,8 @@ File.prototype.parseShebang = function (code) {
 File.prototype.addImport = function (id, source) {
   var specifiers = [t.importSpecifier(t.identifier("default"), id)];
   var declar = t.importDeclaration(specifiers, t.literal(source));
-  this.unshiftProgramAtEnd.push(declar);
+  declar._blockHoist = 3;
+  this.dynamicImports.push(declar);
 };
 
 File.prototype.addDeclaration = function (name) {
@@ -231,11 +232,6 @@ File.prototype.transform = function (ast) {
 
   _.each(this.transformers, function (transformer) {
     transformer.transform(self);
-    var unshift = self.unshiftProgramAtEnd;
-    if (unshift.length) {
-      ast.program.body = unshift.concat(ast.program.body);
-      self.unshiftProgramAtEnd = [];
-    }
   });
 
   astRun("exit");

lib/6to5/transformation/transformers/es6-modules.js

@@ -1,5 +1,11 @@
 var t = require("../../types");
 
+exports.ast = {
+  before: function (ast, file) {
+    ast.program.body = file.dynamicImports.concat(ast.program.body);
+  }
+};
+
 exports.ImportDeclaration = function (node, parent, file) {
   var nodes = [];
 
@@ -11,6 +17,12 @@ exports.ImportDeclaration = function (node, parent, file) {
     file.moduleFormatter.importDeclaration(node, nodes, parent);
   }
 
+  if (nodes.length === 1) {
+    // inherit `_blockHoist`
+    // this for `_blockHoist` in File.prototype.addImport
+    nodes[0]._blockHoist = node._blockHoist;
+  }
+
   return nodes;
 };