NodeJS run_ API method should return native JavaScript Promise

[devversion]

Currently the run_ method, exposed through the dart.sass.js file,
returns a Future. The Future object is not accessible due to the
Dart compilation process and Future not being a thing in the
JavaScript/NodeJS platform.

In order to provide a better interface to the Sass API for
JavaScript/NodeJS consumers, a native Promise is now returned.

Fixes bazelbuild/rules_sass#96.

[devversion]

I tried figuring out if the test failures are due to changes in this PR, but it doesn't look like it. I sent an empty test PR to confirm this: #933

[jathak]

It looks like tests are failing because the CIs have started to use a version of the source_span package with slightly different output from our current specs. This should be resolved once #926 is merged.

[nex3]

We're in the process of transitioning the way we build JS executables to use cli_pkg (#924), so it's probably a good idea to update this there rather than in this package.

[jelbourn]

Would someone from the Sass team be able to follow up on that? The Angular folks can only go so deep into the dart ecosystem before we're over our heads

[nex3]

I can, but it'll have to be after we do this migration. Can you file an issue on http://github.com/google/dart_cli_pkg?

[devversion]

Just came across this again. I've created the issue on the dart_cli_pkg and also experimented a bit with it, but looks like it will be time consuming to set up the project/tests on my end: google/dart_cli_pkg#50