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

sdk/lib/io/http.dart

Index: sdk/lib/io/http.dart
diff --git a/sdk/lib/io/http.dart b/sdk/lib/io/http.dart
index 912a366d737af66da4a3bdca182ba71b277fcba2..9a54af20c27d0421de5a5aebcb119a3653b36218 100644
--- a/sdk/lib/io/http.dart
+++ b/sdk/lib/io/http.dart
@@ -866,10 +866,64 @@ abstract class HttpClient {
    * separated by semicolons, e.g.
    *
    *   "PROXY host:port; PROXY host2:port2; DIRECT"
+   *
+   * The static function [findProxyFromEnvironment] on this class can
+   * be used to implement proxy server resolving based on environment
+   * variables.
    */
   set findProxy(String f(Uri url));
 
   /**
+   * Function for resolving the proxy server to be used for a HTTP
+   * connection from the proxy configuration specified through
+   * environment variables.
+   *
+   * The following environment variables are taken into account:
+   *
+   *  * http_proxy
+   *  * https_proxy
+   *  * no_proxy
+   *  * HTTP_PROXY
+   *  * HTTPS_PROXY
+   *  * NO_PROXY
+   *
+   * [:http_proxy:] and [:HTTP_PROXY:] specify the proxy server to use for
+   * http:// urls. Use the format [:hostname:port:]. If no port is used a
+   * default of 1080 will be used. If both are set the lower case one takes
+   * precedence.
+   *
+   * [:https_proxy:] and [:HTTPS_PROXY:] specify the proxy server to use for
+   * https:// urls. Use the format [:hostname:port:]. If no port is used a
+   * default of 1080 will be used. If both are set the lower case one takes
+   * precedence.
+   *
+   * [:no_proxy:] and [:NO_PROXY:] specify a comma separated list of
+   * postfixes of hostnames for which not to use the proxy
+   * server. E.g. the value "localhost,127.0.0.1" will make requests
+   * to both "localhost" and "127.0.0.1" not use a proxy. If both are set
+   * the lower case one takes precedence.
+   *
+   * To activate this way of resolving proxies assign this function to
+   * the [findProxy] property on the [HttpClient].
+   *
+   *   HttpClient client = new HttpClient();
+   *   client.findProxy = HttpClient.findProxyFromEnvironment;
+   *
+   * If you don't want to use the system environment you can use a
+   * different one by wrapping the function.
+   *
+   *   HttpClient client = new HttpClient();
+   *   client.findProxy = (url) {
+   *     return HttpClient.findProxyFromEnvironment(
+   *         url, {"http_proxy": ..., "no_proxy": ...});
+   *   }
+   */
+  static String findProxyFromEnvironment(Uri url,
+                                         {Map<String, String> environment}) {
+    return _HttpClient._findProxyFromEnvironment(url, environment);
+  }
+
+  /**
    * Shutdown the HTTP client. If [force] is [:false:] (the default)
    * the [:HttpClient:] will be kept alive until all active
    * connections are done. If [force] is [:true:] any active