Process outstanding events before making a new HTTP client request

Process outstanding events before making a new HTTP client request

This fixes the issue of using a HTTP client connection from the
connection pool which was closed and where the close indication has
already been received by the event handler.

This specifically fixes the 'pub get' issue where a long running
constraint resolver would end with the error "Connection closed before
full header was received"

Retrying GET methods when the connection is closed after succesfuly
sending (parts of) the request is still missing.

R=kustermann@google.com

BUG=http://dartbug.com/21798

Committed: https://code.google.com/p/dart/source/detail?r=45127

[kustermann, 2015-04-09 14:32:30]

lgtm

- do we have a reproducable test case?
- is 'new Future(() {})' using microtasks or Timer.run()?

[Søren Gjesse, 2015-04-14 08:24:43]

On 2015/04/09 14:32:30, kustermann wrote:
> lgtm
> 
> - do we have a reproducable test case?
> - is 'new Future(() {})' using microtasks or Timer.run()?

If fixes the concrete issue specified in the bug (though that might still fail
due to timing of the network packages). It also fixes the 'pub get' fail when
the resolver is running for a long time.

We still need to add retrying of at least GET requests, so I will keep the bug
open.

new Future(...) is using Timer.run - it is specified in the documentation.

[Søren Gjesse, 2015-04-14 08:25:03]

Committed patchset #1 (id:1) manually as 45127 (presubmit successful).