Package map entries can contain relative paths.
See related commit: d7d99c62

Updating base turns out to be more complicated than I thought.

This reverts commit daa822aa.
This reverts commit c38428c7.
This reverts commit 4b73a8a0.

Update base, gn, and their clients.

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

Fix uri canonicalization in DartLibraryProviderFiles

SkyShell now supports --packages instead of --package-root.

We don't use this anymore.

We now use flutter.mojo instead.

Also, in testing, crash when we can't load a library off disk.

This updates to mojo commit d259eb58aa59 and limits the roll script to
only pull in the parts of //mojo that are currently being used. More
stuff will be dropped in the future.

Now we print Dart_GetError only when we don't have a stack trace. Also, improve
the usability of the sky_shell command line tool by defaulting to "packages"
for the package-root and not crashing when we don't have a package-root until
we actually try to load a package. Finally, remove console spam when we fail to
bind the observatory port.

Fixes #689

We don't want to support file URLs in general, but having them in the "files"
version of the library provider makes it easier to integrate with package:test.

- Add missing dependencies caught by gn's new stricter checker
- Update opentype code to use new OTS API. This code is updated from HEAD
  Blink.
- Add base dependency to wtf to avoid redefining marcos from base/macros.h
- Update callers of various base string utility functions to use base
  namespace.