[dart2wasm] Fix minor bug in set.
[dart2wasm] Fix minor bug in set.

Change-Id: I127539ae7c99d9b5ef519ed10077f33f019504b1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251502
Commit-Queue: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>
Reviewed-by: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk