Ignore the UnmodifiableUint8ListView deprecation warning (#915)

We can't fix it until the minimum supported SDK version has the
replacement. Ignore the warning for now.

Author: osa1

protobuf/test/coded_buffer_reader_test.dart

@@ -84,6 +84,10 @@ void main() {
     });
 
     test('unmodifiable-uint8-list-view', () {
+      // TODO: Use `Uint8List.asUnmodifiableView` instead of
+      // `UnmodifiableUint8ListView` when it's available in the oldest
+      // supported SDK version.
+      // ignore: deprecated_member_use
       testWithList(UnmodifiableUint8ListView(Uint8List.fromList(inputBuffer)));
     });