[BUG][FIX] Scamper doesn't start in 3.7a-5526 ( [er] should be split into Scamper/FileUrl pieces )

[ned at squeakland.org]

I tried loading this into an image that didn't have Scamper loaded, and
got a walkback because it included changes to the class FileInput, which
didn't exist. UrlTest also doesn't exist in this image.

Perhaps this change set (which is important independent of Scamper
because of its changes to FileUrl) should be separated into two or more
parts.















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