Add support for the metadata we currently support in comments

Add support for the metadata we currently support in comments
Committed: https://code.google.com/p/dart/source/detail?r=11230

[Brian Wilkerson, 2012-08-22 20:28:54]

We (the editor team) would like to add a library to contain the definition of
metadata that can be used to annotate code for the purpose of improving the
tooling support. We think it's important that this be defined as part of the SDK
in order to allow all of the tools to use the same definitions.

I chose the two examples in this CL because we have already provided support for
them in Editor when embedded in comments, and we would like to have a clear
migration path for user's using the old format so that we can remove the old
support. Our intention is to add additional metadata only when there is a clear
need for it.

Please let me know if you think this belongs somewhere other than the SDK. Also
let me know if there are style violations as this is the first serious piece of
Dart code that I've written.

[Bob Nystrom, 2012-08-22 20:41:57]

LGTM.

It would be nice if there was an annotation to annotate that a constant is
intended to be used as an annotation (say that five times fast). Like:

@annotation
const deprecated = const _Deprecated();

http://codereview.chromium.org/10877024/diff/1/pkg/meta/meta.dart
File pkg/meta/meta.dart (right):

http://codereview.chromium.org/10877024/diff/1/pkg/meta/meta.dart#newcode26
pkg/meta/meta.dart:26: * setter) as one that is intended to be overriding an
inherited class member.
"one that is intended to be overriding" -> "overriding"

[Brian Wilkerson, 2012-08-22 21:16:01]

> It would be nice if there was an annotation to annotate that a constant is
> intended to be used as an annotation (say that five times fast). Like:

Presumably so that tools could warn you if you're using a constant that is
intended to be an annotation somewhere outside of metadata. What I'm not sure of
is whether code that uses the mirrors API would need to reference the constants
in order to determine which annotations were associated with an element, and if
so whether this kind of warning would just be annoying. Thoughts?

[devoncarew, 2012-08-22 22:45:32]

lgtm

[dgrove, 2012-08-23 00:25:34]

lgtm

[Brian Wilkerson, 2012-08-23 14:52:51]

http://codereview.chromium.org/10877024/diff/1/pkg/meta/meta.dart
File pkg/meta/meta.dart (right):

http://codereview.chromium.org/10877024/diff/1/pkg/meta/meta.dart#newcode26
pkg/meta/meta.dart:26: * setter) as one that is intended to be overriding an
inherited class member.
Done.