Add custom export declaration fn for AMD modules

lib/6to5/transformation/modules/amd.js

@@ -96,6 +96,39 @@ AMDFormatter.prototype.importSpecifier = function (specifier, node, nodes) {
   ]));
 };
 
+AMDFormatter.prototype.exportDeclaration = function (node, nodes) {
+  if (node.default) {
+    var declar = node.declaration;
+    var assign;
+
+    // exports = VALUE;
+    var templateName = "exports-default";
+
+    if (t.isFunction(declar) || !this.hasNonDefaultExports) {
+      assign = util.template(templateName, {
+        VALUE: this._pushStatement(declar, nodes)
+      }, true);
+
+      // hoist to the top if this default is a function
+      nodes.push(this._hoistExport(declar, assign, 3));
+      return;
+    } else {
+      // this export isn't a function so we can't hoist it to the top so we need to set it
+      // at the very end of the file with something like:
+      //
+      //   module.exports = Object.assign(exports["default"], exports)
+      //
+
+      assign = util.template("amd-export-default-assign", true);
+      assign._blockHoist = 0;
+
+      nodes.push(assign);
+    }
+  }
+
+  DefaultFormatter.prototype.exportDeclaration.apply(this, arguments);
+};
+
 AMDFormatter.prototype.exportSpecifier = function (specifier, node, nodes) {
   var self = this;
   return this._exportSpecifier(function () {

lib/6to5/transformation/templates/amd-export-default-assign.js

@@ -0,0 +1 @@
+exports = Object.assign(exports["default"], exports);

lib/6to5/transformation/templates/exports-default.js

@@ -0,0 +1 @@
+exports = VALUE;