Make it an error to call close twice on a file using the async API.

Make it an error to call close twice on a file using the async API.

This is to make it consistent with the sync API.

Additionally, fix situations where a non-exception was passed to
error handlers on File objects.

R=sgjesse@google.com
BUG=dartbug.com/2451
TEST=standalone/io/File*

Committed: https://code.google.com/p/dart/source/detail?r=6564

[Søren Gjesse, 2012-04-16 11:14:12]

lgtm