One more error code to check.

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index f88f130cac224d4754c81cd3e6a71451cc1fd62d..c622b9505ddc0b81ca2d1a5ff8d0526f0b02d519 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -326,7 +326,9 @@ Future<String> httpGetString(uri) {
   connection.onError = (e) {
     // Show a friendly error if the URL couldn't be resolved.
     if (e is SocketIOException &&
-        (e.osError.errorCode == 8 || e.osError.errorCode == -2)) {
+        (e.osError.errorCode == 8 ||
+         e.osError.errorCode == -2 ||
+         e.osError.errorCode == -5)) {
       e = 'Could not resolve URL "${uri.origin}".';
     }