await for ... on ... catch syntax

There is currently no way that I am aware of to handle stream errors in `await for` loops without breaking the loop.

```dart
/// This works, but any error will break the loop and close the subscription
try {
await for (var e in service.probeEvents()) {
status = status.copyWith(probes: e);
yield status;
}
} on StreamException {
///
} catch (e) {
///
}
```

I am proposing that we add syntax on `await for` as an added feature.

```dart
/// Proposing this syntax, where errors will not break the loop
await for (var e in service.probeEvents()) {
status = status.copyWith(probes: e);
yield status;
} on StreamException {
///
} catch (e) {
///
} finally {
/// onDone
}
```

Start by reviewing the issue's proposed await-for syntax and the existing error-handling behavior it describes. The issue names no files or tests; done would require a language-design decision on whether the syntax and its stream-error semantics should be adopted.

Written by the indexing model from the issue text.