Add support for comments - line and doc

[kevmoo]

[srawlins]

Something to keep in mind for this one: supporting references in doc comments. For example, if I have:

import 'foo.dart' as _i1;

/// This class extends [Foo].
class Bar extends _i1.Foo {}

Then I get a "Only reference in scope identifiers in doc comments" lint report. If a first-class Doc comment could be made, I could add text ("This class extends ") and references (TypeReference(...)).

[donny-dont]

@srawlins if you take another stab at this my wish list would be

• Add line breaks to support formatting line lengths
• Some markdown support, like adding a link to a URL
• Example code block

[srawlins]

Good ideas. I don't foresee mailing a PR any time soon.

[devoncarew]

dart-archive/code_builder#389 implements a little bit of this - the ability to have line comments for libraries, emitted at the beginning of the file (// Generated by ...).