Fix iOS build which broke after the change to fuse --strong option into

--preview-dart-2.

packages/flutter_tools/bin/xcode_backend.sh

@@ -93,11 +93,6 @@ BuildApp() {
     preview_dart_2_flag="--preview-dart-2"
   fi
 
-  local strong_flag=""
-  if [[ -n "$STRONG" ]]; then
-    strong_flag="--strong"
-  fi
-
   if [[ "$CURRENT_ARCH" != "x86_64" ]]; then
     local aot_flags=""
     if [[ "$build_mode" == "debug" ]]; then
@@ -113,7 +108,6 @@ BuildApp() {
       ${aot_flags}                                                          \
       ${local_engine_flag}                                                  \
       ${preview_dart_2_flag}                                                \
-      ${strong_flag}                                                        \
 
     if [[ $? -ne 0 ]]; then
       EchoError "Failed to build ${project_path}."
@@ -146,7 +140,6 @@ BuildApp() {
     ${precompilation_flag}                                                \
     ${local_engine_flag}                                                  \
     ${preview_dart_2_flag}                                                \
-    ${strong_flag}                                                        \
 
   if [[ $? -ne 0 ]]; then
     EchoError "Failed to package ${project_path}."

packages/flutter_tools/lib/src/ios/xcodeproj.dart

@@ -56,7 +56,6 @@ void updateXcodeGeneratedProperties({
 
   if (previewDart2) {
     localsBuffer.writeln('PREVIEW_DART_2=true');
-    localsBuffer.writeln('STRONG=true');
   }
 
   // Add dependency to CocoaPods' generated project only if plugins are used.