flutter docs generated from head don't know about Dart Core

[jcollins-g]

Since the analysis driver transition in #1598 it looks like Flutter docs call Dart "Sky Engine" instead of recognizing those libraries are the dart SDK. Probably something wrong with how we're doing embedded SDKs again -- that was it last time this happened (#1107).

[jcollins-g]

This was found thanks to the development of #1600. P1 as it is a minor display wart impacting the front page with high impact.

[devoncarew]

/cc @scheglov - he may know about whether the driver handles the _embedder.yaml extensions (https://github.com/flutter/engine/blob/master/sky/packages/sky_engine/lib/_embedder.yaml).

[scheglov]

The driver gets already created SourceFactory, with SDK and packages configured.

I see that we handle _embedder.yaml in ContextBuilder.

[jcollins-g]

We do something similar in dartdoc too, but apparently what we needed to do changed subtly (and because I didn't have #1600 yet I didn't notice it right away).

[jcollins-g]

Changing to P0, because this also impacts navigation as canonicalization loses its mind when it doesn't have an SDK. Dartdoc emits hundreds of warnings and mislinks things due to canonicalization errors. Fortunately, we haven't shipped a release with the driver change yet.