fix es3 compatibility for exports default

babel-fix-exports-default

ref: http://www.aliued.com/?p=3240

// babel-template.js file, line 124 to add below:

if (replacement) {
      replacement[TEMPLATE_SKIP] = true;
      path.replaceInline(replacement);

      // fix es3 compatibility:
      // exports.default -> exports["default"]
      var pnode = path.parent
      if(t.isMemberExpression(pnode)){
        var prop = pnode.property;
        if (!pnode.computed && t.isIdentifier(prop) && !t.isValidIdentifier(prop.name)) {
            pnode.property = t.stringLiteral(prop.name);
            pnode.computed = true;
          }
      }

    }