🚑️ Fix type promotions for the UTF-8 encoder (#2185)

Fixes #2184

.github/workflows/tests.yml

@@ -93,6 +93,8 @@ jobs:
         run: melos run test:web:firefox
       - name: '[Verify step] Test Flutter packages'
         run: melos run test:flutter
+      - name: '[Verify step] Build Flutter APK'
+        run: melos run build:example:apk
       # Coverage
       - name: '[Coverage] Format & print test coverage'
         if: ${{ matrix.sdk == 'stable' }}

dio/CHANGELOG.md

@@ -5,7 +5,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 
 ## Unreleased
 
-*None.*
+- Fix type promotions for the UTF-8 encoder on previous Dart SDKs.
 
 ## 5.4.3
 

dio/lib/src/form_data.dart

@@ -172,26 +172,32 @@ class FormData {
     _isFinalized = true;
 
     final controller = StreamController<Uint8List>(sync: false);
-    void writeAscii(String s) => controller.add(utf8.encode(s));
-    void writeUtf8(String string) => controller.add(utf8.encode(string));
+
+    void writeUtf8(String s) {
+      final encoded = utf8.encode(s);
+      controller.add(
+        encoded is Uint8List ? encoded : Uint8List.fromList(encoded),
+      );
+    }
+
     void writeLine() => controller.add(_rnU8); // \r\n
 
     for (final entry in fields) {
-      writeAscii('--$boundary$_rn');
-      writeAscii(_headerForField(entry.key, entry.value));
+      writeUtf8('--$boundary$_rn');
+      writeUtf8(_headerForField(entry.key, entry.value));
       writeUtf8(entry.value);
       writeLine();
     }
 
     Future<void>(() async {
       for (final file in files) {
-        writeAscii('--$boundary$_rn');
-        writeAscii(_headerForFile(file));
+        writeUtf8('--$boundary$_rn');
+        writeUtf8(_headerForFile(file));
         await writeStreamToSink(file.value.finalize(), controller);
         writeLine();
       }
     }).then((_) {
-      writeAscii('--$boundary--$_rn');
+      writeUtf8('--$boundary--$_rn');
     }).whenComplete(() {
       controller.close();
     });

melos.yaml

@@ -47,6 +47,7 @@ scripts:
   httpbun:com:
     description: Run httpbun locally
     run: echo "const httpbunBaseUrl = 'https://httpbun.com';" > dio_test/lib/src/httpbun.dart
+
   test:
     name: All tests
     run: |
@@ -120,6 +121,12 @@ scripts:
       melos run test
       melos run coverage:format
       melos run coverage:show
+
+  build:example:apk:
+    run: |
+      cd example_flutter_app
+      flutter build apk --debug
+
   coverage:format:
     name: Format coverage
     run: |