Change a TODO to a permanent comment (#114)

https://github.com/dart-lang/sdk/issues/19566 is closed and no changes
to the behavior are planned. Remove the TODO since this won't ever be
actionable, and change to an explanatory comment.
diff --git a/lib/src/chain.dart b/lib/src/chain.dart
index 8a03b6c..628b8fa 100644
--- a/lib/src/chain.dart
+++ b/lib/src/chain.dart
@@ -96,7 +96,8 @@
       try {
         return callback();
       } on Object catch (error, stackTrace) {
-        // TODO(nweiz): Don't special-case this when issue 19566 is fixed.
+        // Forward synchronous errors through the async error path to match the
+        // behavior of `runZonedGuarded`.
         Zone.current.handleUncaughtError(error, stackTrace);
 
         // If the expected return type of capture() is not nullable, this will