Fixes dart-lang#2021

In this feature, we list extension members of an extended type on that extended
type's page. The members are listed inline with other instance members and are
cross-linked over to the extension's member pages.

* Introduce `availableInstanceMethods` as a list combining both instance
  methods declared in a container, and instance methods declared in an
  applicable extension. The same for `availableInstanceOperators` and
  `availableInstanceFields`.
* Introduce some constructors like `Field.providedByExtension` for members that
  are provided by an extension.
* Add lots of documentation clarifying _where_ various members come from.
* We introduce a new getter on ElementType, `nameWithGenericsPlain`, which is
  not HTML-formatted, for use in HTML "title text."