Add HTTP proxy tunnel support

Add HTTP proxy tunnel support

The HTTP client will now use the CONNECT method with a proxy
server for all HTTPS requests. The CONNECT method will establish
a tunnel and a TLS end-to-end connection will then be created
through that tunnel.

The previous method of sending a HTTPS url as the request URI is no
longer supported. Also it is not possible to configure normal HTTP
requests to go through a tunnel created with CONNECT. All HTTP
requests use the normal procedure when going through a proxy.

Add the ability to specify a host name when creating a secure
sonnection on top of a non-secure one. This is required for a
proxy tunnel as the initial connection is to a different host as
the one the TLS connection will be with.

R=ager@google.com
BUG=https://code.google.com/p/dart/issues/detail?id=5468

Committed: https://code.google.com/p/dart/source/detail?r=22119

[Mads Ager (google), 2013-04-29 10:05:01]

LGTM

https://codereview.chromium.org/14493015/diff/1/sdk/lib/io/http_impl.dart
File sdk/lib/io/http_impl.dart (right):

https://codereview.chromium.org/14493015/diff/1/sdk/lib/io/http_impl.dart#new...
sdk/lib/io/http_impl.dart:1335: if (uri.domain.isEmpty || (uri.scheme != "http"
&& uri.scheme != "https")) {
long line

[Søren Gjesse, 2013-04-29 13:30:22]

Committed patchset #1 manually as r22119 (presubmit successful).