Force SourceSpan.url to be absolute in the JS API (#2298)

Partially addressed #3908

Author: nex3

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 1.78.0
 
+### JS API
+
+* Fix a bug where accessing `SourceSpan.url` would crash when a relative URL was
+  passed to the Sass API.
+
 ### Embedded Sass
 
 * Explicitly expose a `sass` executable from the `sass-embedded` npm package.

lib/src/js/source_span.dart

@@ -2,6 +2,7 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'package:path/path.dart' as p;
 import 'package:source_span/source_span.dart';
 
 import '../util/lazy_file_span.dart';
@@ -21,7 +22,8 @@ void updateSourceSpanPrototype() {
     getJSClass(item).defineGetters({
       'start': (FileSpan span) => span.start,
       'end': (FileSpan span) => span.end,
-      'url': (FileSpan span) => span.sourceUrl.andThen(dartToJSUrl),
+      'url': (FileSpan span) => span.sourceUrl.andThen((url) => dartToJSUrl(
+          url.scheme == '' ? p.toUri(p.absolute(p.fromUri(url))) : url)),
       'text': (FileSpan span) => span.text,
       'context': (FileSpan span) => span.context,
     });