Fix #905: Show methods the optimizer worked on if it crashes

The error message will look like this:

    The Scala.js optimizer crashed while optimizing sjs_test_jsinterop_UndefOrTest$$anonfun$1$$anonfun$apply$mcV$sp$7.init___sjs_test_jsinterop_UndefOrTest$$anonfun$1
    Methods attempted to inline:
    * sr_AbstractFunction0$mcV$sp.init___
    * sr_AbstractFunction0.init___
    * O.init___
Must be a `val`. It's never assigned.