Wasn't sure where to place this issue - here or in the App Scripts repo. I placed it there - sorry to duplicate
FIXED - I had Firebase as a dependency as well as AngularFire - removing Firebase from package.json fixed the issue.
I don't think this is a long term solution but if you are trying to get 'ionic serve' to work and this is your error you can make a change in a typescript file and save it and then it starts webpack and works. Well works for me and I'm using angularfire2 2.0.0-beta.6 and firebase 3.6.4
"Delete the firebase.d.ts in the folder node_modules\firebase"
https://github.com/driftyco/ionic-cli/issues/1713#issuecomment-267753479
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"Delete the firebase.d.ts in the folder node_modules\firebase"
https://github.com/driftyco/ionic-cli/issues/1713#issuecomment-267753479