[dart2wasm] Remove externref and use the new value for anyref.
[dart2wasm] Remove externref and use the new value for anyref.

This matches the spec after anyref and externref were merged, and it's
necessary in order to use the latest Binaryen, which has removed support
for the old value.

Change-Id: I7f1bb5cb08637dd79fe975603d8e236d73ca4fdc
Cq-Include-Trybots: luci.dart.try:dart2wasm-linux-x64-d8-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245375
Reviewed-by: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>
Commit-Queue: 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