Don't ignore the ByteData buffer offset in StringCodec.decode(). (#21011)

packages/flutter/lib/src/services/message_codecs.dart

@@ -41,7 +41,7 @@ class StringCodec implements MessageCodec<String> {
   String decodeMessage(ByteData message) {
     if (message == null)
       return null;
-    return utf8.decoder.convert(message.buffer.asUint8List());
+    return utf8.decoder.convert(message.buffer.asUint8List(message.offsetInBytes, message.lengthInBytes));
   }
 
   @override

packages/flutter/test/services/message_codecs_test.dart

@@ -24,6 +24,19 @@ void main() {
       _checkEncodeDecode<String>(string, 'hello');
       _checkEncodeDecode<String>(string, 'special chars >\u263A\u{1F602}<');
     });
+    test('ByteData with offset', () {
+      const MessageCodec<String> string = StringCodec();
+      final ByteData helloWorldByteData = string.encodeMessage('hello world');
+      final ByteData helloByteData = string.encodeMessage('hello');
+
+      final ByteData offsetByteData = new ByteData.view(
+          helloWorldByteData.buffer,
+          helloByteData.lengthInBytes,
+          helloWorldByteData.lengthInBytes - helloByteData.lengthInBytes
+      );
+
+      expect(string.decodeMessage(offsetByteData), ' world');
+    });
   });
   group('JSON message codec', () {
     const MessageCodec<dynamic> json = JSONMessageCodec();