Fix hang when opening sockets on ports that are not available.

runtime/bin/socket_impl.dart

Index: runtime/bin/socket_impl.dart
diff --git a/runtime/bin/socket_impl.dart b/runtime/bin/socket_impl.dart
index 97c3e5e63fe6a2f994dfb98fee6a1921a6e63f97..10cc29585c4ce6cd50c5ae4642ddd08475c4c6ed 100644
--- a/runtime/bin/socket_impl.dart
+++ b/runtime/bin/socket_impl.dart
@@ -201,6 +201,7 @@ class _ServerSocket extends _SocketBase implements ServerSocket {
   factory _ServerSocket(String bindAddress, int port, int backlog) {
     ServerSocket socket = new _ServerSocket._internal();
     if (!socket._createBindListen(bindAddress, port, backlog)) {
+      socket.close();
       return null;
     }
     if (port != 0) {
@@ -247,6 +248,7 @@ class _Socket extends _SocketBase implements Socket {
   factory _Socket(String host, int port) {
     Socket socket = new _Socket._internal();
     if (!socket._createConnect(host, port)) {
+      socket.close();
       return null;
     }
     return socket;