Security: do not leak info to error handlers

[DartBot]

This issue was originally filed by antonm@google.com

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<quote>
I’m able to read redirects on error handlers, as well as the first line of code by listening on Syntax Errors:
http://go/pastebin/2066011
</quote>

[DartBot]

This comment was originally written by antonm@google.com

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Added Fixed label.