Add support for declarations

This helps guard against skew when new formatting logic is released in
the dart_style package before it's available in a stable SDK release.

This ensures that they work independently of whether the files have
been formatted or not.

Always use the SDK version of the formatter

…2487)

Previously we were loading the version variable after running `npm
install sass@$version`, which caused the local path dependency not to
be overwritten and consequently made the sass-parser package
impossible to install.

Add support for `@if` and `@else`

Previously this deployed an empty package.

Closes #2485

In the pure JS release, these weren't working because the Dart-to-JS
logger didn't know to pass along the deprecation metadata when falling
back to the wrapped StderrLogger. In the embedded release, these
weren't working because we didn't give it an update to match the
stderr logger.