Allow a `library` directive without a name

[xvrh]

Several tools depend on the ability to add some metadata (@annotation) at a file level.

With the test package you can write

@Skip()
library my_test;

import 'package:test/test.dart';

Or

@Skip()

import 'package:test/test.dart';

The second form, is kind of a hack. The @Skip() attribute is in fact attached to the first import directive.
It causes problems with auto-formatters that re-order or remove imports. (See dart-lang/test#517)

The first form is safer but it requires to write a name for the library.
The linter recommends the name to be of the form package_name.test.path.to.file (http://dart-lang.github.io/linter/lints/package_prefixed_library_names.html).
This rule is necessary to avoid a warning because 2 libraries have the same name.
But this is tedious to write and to maintain.

I would like to write either:

@Skip()
library;

import 'package:test/test.dart';

Or that the analyzer doesn't report warnings if 2 libraries are named _. So I can write

@Skip()
library _;

import 'package:test/test.dart';

[lrhn]

I think it's a very reasonable feature (and so simple I'll label it as both feature and request).

We already have special-casing in DartDoc where it considers a doc-comment on the first import as really being a library documentation. This would allow documentation on the library without having to name the library.

[evanweible-wf]

@lrhn Any update on this? If it is indeed simple to implement, I think it'd be an awesome quality-of-life improvement.

[lrhn]

Duplicate of #1073