Allow functions to be used directly as metadata annotations

I haven't actually looked at the language spec to know for sure whether this is considered a bug or not, but this definitely seems like some unwanted behavior. Filing here instead of `sdk` since I'm guessing it is due to the language specification.

```dart
void foo() {}
const bar = foo;

class Metadata {
const Metadata(this.f);
final void Function() f;
}

// @foo // fails to compile, but this seems like it should
@bar // ok, despite just being = foo
@Metadata(foo) // ok
void foobar() {}
```

Using functions as metadata annotations will be useful for when static metaprogramming roles around.

Start with the Dart language specification rules for metadata annotations and the reproducer in the issue, comparing direct `@foo` with `@bar` and `@Metadata(foo)`. Determine the intended specification change and confirm that the accepted behavior consistently allows functions to be used directly as metadata annotations.

Written by the indexing model from the issue text.