Dart-sass

* No user-visible changes.

* Produce better errors when expected tokens are missing before a closing brace.

* Avoid crashing when compiling a non-partial stylesheet that exists on the
filesystem next to a partial with the same name.

Command-Line Interface

* Add support for the `--watch`, which watches for changes in Sass files on the
filesystem and ensures that the compiled CSS is up-to-date.

* When using `--update`, surface errors when an import doesn't exist even if the
file containing the import hasn't been modified.

* When compilation fails, delete the output file rather than leaving an outdated
version.

* Fix a bug where an absolute Windows path would be considered an `input:output`
pair.

* Forbid custom properties that have no values, like `--foo:;`, since they're
forbidden by the CSS spec.

* Fix a bug where an importer would be passed an incorrectly-resolved URL when
handling a relative import.

* Throw an error when an import is ambiguous due to a partial and a non-partial
with the same name, or multiple files with different extensions. This matches
the standard Sass behavior.

Command-Line Interface

* Add an `--interactive` flag that supports interactively running Sass
expressions (thanks to [Jen Thakar][]!).

[Jen Thakar]: https://github.com/jathak

* Improve the error message for invalid semicolons in the indented syntax.

* Properly disallow semicolons after declarations in the indented syntax.

Command-Line Interface

* Add support for compiling multiple files at once by writing
`sass input.scss:output.css`. Note that unlike Ruby Sass, this *always*
compiles files by default regardless of when they were modified.

This syntax also supports compiling entire directories at once. For example,
`sass templates/stylesheets:public/css` compiles all non-partial Sass files
in `templates/stylesheets` to CSS files in `public/css`.

* Add an `--update` flag that tells Sass to compile only stylesheets that have
been (transitively) modified since the CSS file was generated.

Dart API

* Add `Importer.modificationTime()` and `AsyncImporter.modificationTime()` which
report the last time a stylesheet was modified.

Node API

* Generate source maps when the `sourceMaps` option is set to a string and the
`outFile` option is not set.

* Add support for `elseif` as an alias of `else if`. This is not an
intentional feature, so using it will cause a deprecation warning. It will be
removed at some point in the future.