[dart2wasm] Handle implicit nullable initializer.
[dart2wasm] Handle implicit nullable initializer.

This is to handle the case where a nullable local is used in a loop. In such a case, if we do not implicitly initialize the variable, it will retain its state across multiple loop iterations.

Fixes lib/convert/html_escape_test

Change-Id: I35d4ebe827e2486e847659b2e6b494a1af2be57f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/267120
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