Redirect curl's stderr to stdout, since it draws its progress bar to stderr (#9497)

bin/internal/update_dart_sdk.sh

diff --git a/bin/internal/update_dart_sdk.sh b/bin/internal/update_dart_sdk.sh
index 198a91a..74cc466 100755
--- a/bin/internal/update_dart_sdk.sh
+++ b/bin/internal/update_dart_sdk.sh
@@ -49,7 +49,7 @@
   mkdir -p -- "$DART_SDK_PATH"
   DART_SDK_ZIP="$FLUTTER_ROOT/bin/cache/dart-sdk.zip"
 
-  curl --progress-bar -continue-at=- --location --output "$DART_SDK_ZIP" "$DART_SDK_URL"
+  curl --progress-bar -continue-at=- --location --output "$DART_SDK_ZIP" "$DART_SDK_URL" 2>&1
   unzip -o -q "$DART_SDK_ZIP" -d "$FLUTTER_ROOT/bin/cache" || {
     echo
     echo "It appears that the downloaded file is corrupt; please try the operation again later."