Addresses the issue seen in https://github.com/dart-lang/sdk/issues/26448.
Ultimately we want a better story for summaries in the presence of embedded SDKs; in the short-term this simply disables them.
The trick (and reason for all of the changes here) is that there is *a lot* of temporal coupling in the initiailization of contexts, sdks, resolvers, etc. With this change, embedder processing is pulled out to where it can be done before the SDK is configured. Hopefully this even makes this a bit more clear as resolver setup is less complex.
Open question: I think the logic that calls `pub list-dirs` is dead and can safely be removed. Comments there welcome.
Thanks!
BUG=
R=brianwilkerson@google.com, scheglov@google.com
Review URL: https://codereview.chromium.org/1984733003 .
Devon Carew