Issue:
  Firefox performs more strict validation when enabling the `js-string`
  builtin. Namely it requires the wasm array to `fromCharCodeArray` to be
  nullable (as per-spec), where D8 is more lenient and allows non-nullable
  wasm array just fine.

  In principle it shouldn't be nullable as passing a null will make the
  function trap at runtime. But the spec says it should be nullable and if
  it isn't, Firefox will fail validation.

  Soon Chrome will start enforcing this.

Fix:
  Import the function with a nullable wasm array type as parameter.

Why Cherry-pick:
  Apps compiled with dart2wasm will no longer work with newer Chrome
  (once they roll out stricter validation).

Risk: Low

Issue #59899

Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/404300
Change-Id: Icde5eccfdc2cf1e4666be6c670105b896b1a19a7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413440
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

Change-Id: I6b935ab445657efddc9fe3eac8f863035be6f00c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414667
Auto-Submit: Alexander Thomas <athom@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>