We had switched to libjpeg, but libjpeg was failing to decode images when
built with the defines in our .gn files

We don't use yasm, libjpeg_turbo, or mojo's devtools.

We don't need to run "pub get" anymore because we don't have any real Dart
packages in this repo anymore.

during precompilation

Update dart revision to include changes required to enable profiling
during precompilation

There's no need to exclude it anymore.

* Remove most of mojo/dart/embedder
  * Update roll.py to only import a subset of mojo/dart/embedder
* Add mojo/dart/observatory
* Update mojo_sdk_revision to get fixes to dart_pkg.py

Reverts 2c10c3a7 manually.
See https://github.com/flutter/engine/pull/1620

Roll Dart runtime forward to:

6aab1cecb25f8e04087320f2082336073628afb4

Roll Observatory pub packages to:

5c199c5954146747f75ed127871207718dc87786

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Update to new isolate create callback signature.