Add onClosed callbacks to HTTP output streams

Add onClosed callbacks to HTTP output streams

The onClosed will e called after the stream is closed, but the actual
data might still be buffered internally.

BUG=dart:7761
R=ager@google.com

Committed: https://code.google.com/p/dart/source/detail?r=16860

[Mads Ager (google), 2013-01-09 15:53:58]

LGTM

[Mads Ager (google), 2013-01-09 16:07:44]

Looking at the documentation of OutputStream, we explicitly say that onClosed is
not called until all data has been written to the underlying communication
channel. So maybe we should do this change after all?

[Mads Ager (google), 2013-01-09 16:08:02]

On 2013/01/09 16:07:44, Mads Ager wrote:
> Looking at the documentation of OutputStream, we explicitly say that onClosed
is
> not called until all data has been written to the underlying communication
> channel. So maybe we should do this change after all?

shouldn't