Spec doesn't specify what happens to ??= with async expressions.

[lrhn]

The spec says

v ??= e is equivalent to the evaluation of the expression ((x) => x == null ? v = e : x)(v) ...

This translation is not correct if e contains an await expression.
In that case, the translation might want to not use a function expression, and be something like:
((x = v) == null ? v = e : x)
where x still fresh and declared somewhere.
This retains the synchronous execution in the non-null case that the expression suggests (if v is not null, the expression, including the await, is not executed).

[lrhn]

(This is a general problem with many of the rewrites in the spec - if they use anonymous functions to introduce a variable, they may also change the async/sync context of the expressions moved into the synthetic function).

[eernstg]

No milestone now: This does not block Dart 2.

Note that this issue is a subset of #25858.

[lrhn]

This has been fixed. It no longer uses a function literal to "desugar" the behavior.