Regression failure upgrading from 2.5.0 to 2.5.1+, webpack fails to transpile new export statement for ES5.

[jbalinski]

We are attempting to upgrade to the latest version of tslib but ran into an issue with IE11. We are targeting ["web", "es5"] in our webpack.config.js. We are also have importHelpers set to true in our tsconfig.json. With this configuration IE11 no longer runs in webpack development mode.

It appears that the breaking change was made in tslib 2.5.1. The following export was added to tslib.es6.mjs:

export default {
  __extends,
  __assign,
  __rest,
  __decorate,
  __param,
  __metadata,
  __awaiter,
  __generator,
  __createBinding,
  __exportStar,
  __values,
  __read,
  __spread,
  __spreadArrays,
  __spreadArray,
  __await,
  __asyncGenerator,
  __asyncDelegator,
  __asyncValues,
  __makeTemplateObject,
  __importStar,
  __importDefault,
  __classPrivateFieldGet,
  __classPrivateFieldSet,
  __classPrivateFieldIn,
};

This ES6 style of export is not transpiled by webpack when running in development mode and which causes the issue in IE11. The issue does not occur when running in production mode. One workaround is to manually include the es5 compliant tslib.js file on our pages and take webpack out of the picture.

[jakebailey]

If you're targeting es5 in your webpack config, then this file should be getting downleveled in syntax by webpack or babel or similar; this file is .mjs after all and is intended to contain ESM syntax like the above. This (to me) sounds like a bug in webpack or an incorrect configuration that isn't downleveling all of the files in a build.