[dart2wasm] Trivial fix for `visitThis` in closures.
[dart2wasm] Trivial fix for `visitThis` in closures.

Change-Id: I4eda2430adb89d328dcec8b18e7acf1652f86c10
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251501
Reviewed-by: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@google.com>
Commit-Queue: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@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