Process outstanding events before making a new HTTP client request

Process outstanding events before making a new HTTP client request

This re-applies r45127, which was reverted in r45129. The revert was due
to a failing test which is re-written here to perform the separate steps
in a deterministic order.

Original change description:

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=lrn@google.com

BUG=http://dartbug.com/21798

Committed: https://code.google.com/p/dart/source/detail?r=45313

[Lasse Reichstein Nielsen, 2015-04-21 12:11:14]

lgtm

https://codereview.chromium.org/1055943004/diff/1/sdk/lib/io/http_impl.dart
File sdk/lib/io/http_impl.dart (right):

https://codereview.chromium.org/1055943004/diff/1/sdk/lib/io/http_impl.dart#n...
sdk/lib/io/http_impl.dart:1886: return new Future(() => connect(new
HttpException("No proxies given")));
Please document why this is being done.

[Søren Gjesse, 2015-04-21 12:15:03]

Committed patchset #2 (id:20001) manually as 45313 (presubmit successful).