Close Ipv4 server when failing to start Ipv6 (flutter#12)

Fixes a bug that could prevent the VM from shutting down if there is a
port conflict with Ipv6 but not Ipv4.

Author: natebosch

CHANGELOG.md

@@ -1,3 +1,8 @@
+## 2.0.6
+
+* If there is a problem starting a loopback Ipv6 server, don't keep the Ipv4
+  server open when throwing the exception.
+
 ## 2.0.5
 
 * Update SDK constraints to `>=2.0.0-dev <3.0.0`.

lib/http_multi_server.dart

@@ -145,14 +145,17 @@ class HttpMultiServer extends StreamView<HttpRequest> implements HttpServer {
       var v6Server = await bind(InternetAddress.loopbackIPv6, v4Server.port);
       return HttpMultiServer([v4Server, v6Server]);
     } on SocketException catch (error) {
+      // If there is already a server listening we'll lose the reference on a
+      // rethrow.
+      await v4Server.close();
+
       if (error.osError.errorCode != _addressInUseErrno) rethrow;
       if (port != 0) rethrow;
       if (remainingRetries == 0) rethrow;
 
       // A port being available on IPv4 doesn't necessarily mean that the same
       // port is available on IPv6. If it's not (which is rare in practice),
       // we try again until we find one that's available on both.
-      await v4Server.close();
       return await _loopback(port, bind, remainingRetries - 1);
     }
   }

pubspec.yaml

@@ -1,5 +1,5 @@
 name: http_multi_server
-version: 2.0.6-dev
+version: 2.0.6
 
 description: A dart:io HttpServer wrapper that handles requests from multiple servers.
 author: Dart Team <[email protected]>