support multiple assignment of export.default in Babel 7

[henryqdineen]

We ran into an odd issue when migrating from Babel 6 to 7 that I tracked down to this plugin. It is specific to Babel 7 and using the same variable multiple times in a ExportNamedDeclaration. I understand that this is plugin is deprecated and don't expect new version but I thought I would leave this here for posterity.

Consider this code here:

const foo = 'bar';
export { foo, foo as default };`

In Babel 6 it would be transpiled as:

Object.defineProperty(exports, "__esModule", {
  value: true
});
var foo = 'bar';
exports.foo = foo;
exports.default = foo;

In Babel 7 it would be transpiled as:

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = exports.foo = void 0;
var foo = 'bar';
exports.default = exports.foo = foo;

The issue I found is that babel-plugin-add-module-exports will incorrectly add the module.exports because the isOnlyExportsDefault() does not correctly handle the case of exports.default = exports.foo = foo;. It only looks at root level expression statements and does not expect this multiple assignment.

My quick fix was to recursively call findExports on the right hand side of the assignment so it can analyze the other exports.

[aminya]

@59naga Could you take a look at this?