A bunch of changes to pave the way for emebdder SDK summary creation.
* Moves _embedder.yaml processing into the embedder SDK (out of the resolver).
* Changes EmbedderUriResolver to compose rather than extend a DartUriResolver.
* Fixes the wierd handling (dropping to add back) of `dart:` URIs.
* Fixes the incorrect overriding docs for `SDKLibraryImpl.shortName`.
* Updates embedder test to include a definition of `dart:core` in anticipation of adding some embedder SDK tests.
Some API work TODO in a follow-up CL (notably the way we check for the presence of embedders by querying the resolver is odd).
BUG=
R=brianwilkerson@google.com
Review URL: https://codereview.chromium.org/2009303002 .
pq
53fe39137f