Support for macros MVP

[srawlins]

In order to minimally support macros, we need a few bits of work. First, we need dartdoc to not crash when it attempts to document a package with macro applications; that should land with #3687.

After that, I'd like these features:

• On the page for a class which has a macro-generated constructor, the constructor is displayed in the main panel and the sidebar, under 'Constructors.'
  • Bonus - it has a little chip saying something like 'macro-generated.' Design TBD.
• Mutatis mutandis for a macro-generated instance method.
• A page for a macro-generated constructor is produced.
• Mutatis mutandis for a macro-generated instance method.
• A macro-generated constructor can be referenced in a doc comment; it results in a valid link in the HTML.
• Mutatis mutandis for a macro-generated instance method.

[srawlins]

After checking 4 of the above 7 boxes, I'm reducing the priority of the remaining work.