Merge pull request #418 from gzm0/sanitize-app

Fix #417: Sanitize App traits (add js.JSApp trait)
Fix #419: Constructor export emits wrong DCE info.
Merge pull request #420 from sjrd/fix-constr-export-dce-info

Fix #419: Constructor export emits wrong DCE info.
Fix #423: Don't add non-js files to ScalaJSClasspath
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