I think this only becomes a problem when adding additional third party libraries.
If I create a blank Ionic 2 project and run Ionic serve everything is as expected.
However, if I run npm install lodash --save then typings install lodash and run ionic serve, I get a Typescript compiler error saying cannot find module lodash. This then stops any app.bundle.js file from being generated.
I don’t think this was a problem before I updated my ionic-cli to the latest version
Cordova CLI: 6.1.1
Gulp version: CLI version 3.9.1
Gulp local: Local version 3.9.1
Ionic Framework Version: 2.0.0-beta.9
Ionic CLI Version: 2.0.0-beta.32
Ionic App Lib Version: 2.0.0-beta.18
ios-deploy version: 1.8.6
ios-sim version: 5.0.8
OS: Mac OS X El Capitan
Node Version: v4.2.4
Xcode version: Xcode 7.3 Build version 7D175
Just posted a comment on another forum post about this too: