use types.isSpecifierDefault in amd modules formatter

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

@@ -78,7 +78,7 @@ AMDFormatter.prototype.importSpecifier = function (specifier, node, nodes) {
 
   if (t.isImportBatchSpecifier(specifier)) {
     // import * as bar from "foo";
-  } else if (specifier.default) {
+  } else if (t.isSpecifierDefault(specifier)) {
     // import foo from "foo";
     ref = t.callExpression(this.file.addDeclaration("interop-require"), [ref]);
   } else {

lib/6to5/types/index.js

@@ -307,3 +307,7 @@ t.inherits = function (child, parent) {
 t.getSpecifierName = function (specifier) {
   return specifier.name || specifier.id;
 };
+
+t.isSpecifierDefault = function (specifier) {
+  return t.isIdentifier(specifier.id) && specifier.id.name === "default";
+};