Use half-close when closing the socking in HttpServerEarlyClientCloseTest.

Use half-close when closing the socking in HttpServerEarlyClientCloseTest.

This should fix the flankyness of the test under Windows. The lesson to
learn from this, is that a Socket.close() is no guaranteed to have sent
all data to the remote peer, even though
Socket.outputStream.onNoPendingWrites have been issued, on Windows.

We might have to visit this later on, if it becomes a pattern other
users fall into.

BUG=http://code.google.com/p/dart/issues/detail?id=2551
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=6868

[Søren Gjesse, 2012-04-23 15:13:34]

lgtm