Minor omission in metadata section of spec

[bwilkerson]

Section 15 (Metadata) currently reads:

Metadata can appear before a library, part header, class, typedef, type parameter,
constructor, factory, function, field, parameter, or variable declaration
and before an import, export or part directive.

but this list is incomplete. It should include 'enum' in the list of declarations.

[gbracha]

Added Accepted label.

[eernstg]

No milestone now: This correction still needs to be performed, but this will not block Dart 2.

Note that dart-lang/sdk#24425 is very similar (and newer); closing that one as a duplicate of this one.

[srawlins]

Also the list needs "static extension."

[lrhn]

And extension types now.
Which suggests to make the text non-normative and refer to the grammar for where metadata can actually occur.

[eernstg]

refer to the grammar

We should definitely do that! Done in #3787.