Handle IPv6 addresses for HTTP client requests

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 da5d2b6bc5c3c37287900d5513315aa0391f92ec..5c49c2f469bf355be603faf03c90e291ca115df6 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -1329,8 +1329,11 @@ class _HttpClientConnection {
                                          proxy,
                                          _httpClient,
                                          this);
+    // For the Host header an IPv6 address must be enclosed in []'s.
+    var host = uri.host;
+    if (host.contains(':')) host = "[$host]";
     request.headers
-        ..host = uri.host
+        ..host = host
         ..port = port
         .._add(HttpHeaders.ACCEPT_ENCODING, "gzip");
     if (_httpClient.userAgent != null) {