Add comment so we don't later delete the "pointless" code.

lib/src/back_end/code_writer.dart

diff --git a/lib/src/back_end/code_writer.dart b/lib/src/back_end/code_writer.dart
index 64d5813..f0f2b8d 100644
--- a/lib/src/back_end/code_writer.dart
+++ b/lib/src/back_end/code_writer.dart
@@ -162,6 +162,8 @@
 
     // If we haven't found an overflowing line yet, then this line might be one
     // so keep track of the unsolved pieces we've encountered on it.
+    // Note: The `isNotEmpty` check is redundant since `addAll()` will do
+    // nothing in that case, but checking it is measurably faster.
     if (!_foundExpandLine && _currentUnsolvedPieces.isNotEmpty) {
       _currentLinePieces.addAll(_currentUnsolvedPieces);
     }