Please add proxy support for http package

[DartBot]

Originally opened as dart-lang/sdk#22453

This issue was originally filed by stasd...@gmail.com

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I was planning to use http package (https://pub.dartlang.org/packages/http) to make requests to remote server.
Unfortunately, my network uses proxy.

While dart's stadard lib's io HttpClient supports proxy, it's not true for http package.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5449880?v=3" align="left" width="48" height="48"hspace="10"> Comment by iposva-google

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Added Pkg-Http, Area-Pkg, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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When running on the VM, the http library uses an HttpClient under the hood. You can instantiate IOClient directly with a custom HttpClient (http://www.dartdocs.org/documentation/http/0.11.1+1/index.html#http/http.IOClient); this allows you to do low-level configuration like proxy logic while still using the nice http API.

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Added AsDesigned label.