Add support for HTTP proxy in the HTTP client

runtime/bin/http.dart

Index: runtime/bin/http.dart
diff --git a/runtime/bin/http.dart b/runtime/bin/http.dart
index 0b9cd30028730ca93c578764076fe4ac3b8505d4..a3f4931c8dc36a568bc6b6d79fcbfa770e163ff4 100644
--- a/runtime/bin/http.dart
+++ b/runtime/bin/http.dart
@@ -709,6 +709,29 @@ abstract class HttpClient {
   HttpClientConnection postUrl(Uri url);
 
   /**
+   * Sets the function used to resolve the proxy server to be used for
+   * opening a HTTP connection to the specified [url]. If this
+   * function is not set, direct connections will always be used.
+   *
+   * The string returned by [f] must be in the format used by browser
+   * PAC (proxy auto-config) scripts. That is either
+   *
+   *   "DIRECT"
+   *
+   * for using a direct connection or
+   *
+   *   "PROXY host:port"
+   *
+   * for using the proxy server [:host:] on port [:port:].
+   *
+   * A configuration can contain several configuration elements
+   * separated by semicolons, e.g.
+   *
+   *   "PROXY host:port; PROXY host2:port2; DIRECT"
+   */
+  set findProxy(String f(Uri url));
+
+  /**
    * Shutdown the HTTP client releasing all resources.
    */
   void shutdown();