Make it an error to await voidExpression

[eernstg]

For the release of Dart 2, it turned out to be a more seriously breaking change than anticipated to make it an error to await e in the case where e has static type void.

Shouldn't we fix this now where null-safety is being introduced?

void foo() => null;

void main() async {
  await foo(); // Error, cannot await an expression of type `void`.
}

@leafpetersen, @lrhn, @munificent, WDYT?

[leafpetersen]

It's fine by me. If @lrhn agrees with this change, we'd need a volunteer to spec it, write tests and file implementation issues.

[lrhn]

I'm always fine with disallowing use of void!

[eernstg]

Spec update in #933.

[eernstg]

Tests added in https://dart-review.googlesource.com/c/sdk/+/144140.

[eernstg]

Created issues for the analyzer and CFE implementation.

[leafpetersen]

Tests and spec have landed, issues filed. I'm going to close this out.