Add a function for finding the HTTP proxy server from environment variables

tests/standalone/io/http_proxy_configuration_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:io";
+import "dart:uri";
+
+expect(expected, String uri, environment) {
+  Expect.equals(expected,
+                HttpClient.findProxyFromEnvironment(Uri.parse(uri),
+                                                    environment: environment));
+}
+
+expectDirect(String uri, environment) {
+  Expect.equals("DIRECT",
+                HttpClient.findProxyFromEnvironment(Uri.parse(uri),
+                                                    environment: environment));
+}
+
+main() {
+  expectDirect("http://www.google.com", {});
+  expect("PROXY www.proxy.com:1080",
+         "http://www.google.com",
+         {"http_proxy": "www.proxy.com"});
+  expect("PROXY www.proxys.com:1080",
+         "https://www.google.com",
+         {"https_proxy": "www.proxys.com"});
+  expect("PROXY www.proxy.com:8080",
+         "http://www.google.com",
+         {"http_proxy": "www.proxy.com:8080"});
+  expect("PROXY www.proxys.com:8080",
+         "https://www.google.com",
+         {"https_proxy": "www.proxys.com:8080"});
+  expect("PROXY www.proxy.com:8080",
+         "http://www.google.com",
+         {"http_proxy": "www.proxy.com:8080",
+          "https_proxy": "www.proxy.com:8080"});
+  expect("PROXY www.proxys.com:8080",
+         "https://www.google.com",
+         {"http_proxy": "www.proxy.com:8080",
+          "https_proxy": "www.proxys.com:8080"});
+  expectDirect("http://www.google.com",
+               {"http_proxy": "www.proxy.com:8080",
+                "no_proxy": "www.google.com"});
+  expectDirect("http://www.google.com",
+               {"http_proxy": "www.proxy.com:8080",
+                "no_proxy": "google.com"});
+  expectDirect("http://www.google.com",
+               {"http_proxy": "www.proxy.com:8080",
+                "no_proxy": ".com"});
+  expectDirect("http://www.google.com",
+               {"http_proxy": "www.proxy.com:8080",
+                "no_proxy": ",,  , www.google.edu,,.com    "});
+  expectDirect("http://www.google.edu",
+               {"http_proxy": "www.proxy.com:8080",
+                "no_proxy": ",,  , www.google.edu,,.com    "});
+  expectDirect("http://www.google.com",
+               {"https_proxy": "www.proxy.com:8080"});
+}