Fix $throwClassCastException in strong mode.
Fix $throwClassCastException in strong mode.

The implementation relied on the implicit conversion of
`instance` to a string in the string concatenation, which is
not valid in strong mode.

mdedetrich/scala-js,spaced/scala-js,gzm0/scala-js,mdedetrich/scala-js,lrytz/scala-js,scala-js/scala-js,japgolly/scala-js,scala-js/scala-js,lrytz/scala-js,mdedetrich/scala-js,sjrd/scala-js,spaced/scala-js,japgolly/scala-js,nicolasstucki/scala-js,SebsLittleHelpers/scala-js,xuwei-k/scala-js,xuwei-k/scala-js,sjrd/scala-js,scala-js/scala-js,spaced/scala-js,nicolasstucki/scala-js,japgolly/scala-js,gzm0/scala-js,sjrd/scala-js,SebsLittleHelpers/scala-js,xuwei-k/scala-js