Quote framework_dir in xcode_backend.sh (flutter#24591)

Author: dnfield

packages/flutter_tools/bin/xcode_backend.sh

@@ -121,12 +121,12 @@ BuildApp() {
   if [[ -e "${project_path}/.ios" ]]; then
     RunCommand rm -rf -- "${derived_dir}/engine"
     mkdir "${derived_dir}/engine"
-    RunCommand cp -r -- ${flutter_podspec} "${derived_dir}/engine"
-    RunCommand cp -r -- ${flutter_framework} "${derived_dir}/engine"
+    RunCommand cp -r -- "${flutter_podspec}" "${derived_dir}/engine"
+    RunCommand cp -r -- "${flutter_framework}" "${derived_dir}/engine"
     RunCommand find "${derived_dir}/engine/Flutter.framework" -type f -exec chmod a-w "{}" \;
   else
     RunCommand rm -rf -- "${derived_dir}/Flutter.framework"
-    RunCommand cp -r -- ${flutter_framework} "${derived_dir}"
+    RunCommand cp -r -- "${flutter_framework}" "${derived_dir}"
     RunCommand find "${derived_dir}/Flutter.framework" -type f -exec chmod a-w "{}" \;
   fi