Fix bug in handling IPv6 proxy hosts

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index de8f832d3862d958aa6f3737d08c6c7480f8aad0..e4960fa5f134657d5caf88ac8115b35c71172b3c 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -2201,7 +2201,7 @@ class _ProxyConfiguration {
           }
           String host = proxy.substring(0, colon).trim();
           if (host.startsWith("[") && host.endsWith("]")) {
-            host = host.substring(1, host.length - 2);
+            host = host.substring(1, host.length - 1);
           }
           String portString = proxy.substring(colon + 1).trim();
           int port;