Fix root-relative @import URLs as passed to importers (#1371)

Closes #1137

CHANGELOG.md

@@ -6,6 +6,10 @@
   relying on this bug, but if so they can simply add `!global` to the variable
   declaration to preserve the old behavior.
 
+* **Potentially breaking bug fix:** Fix a bug where imports of root-relative
+  URLs (those that begin with `/`) in `@import` rules would be passed to
+  both Dart and JS importers as `file:` URLs.
+
 ## 1.35.1
 
 * Fix a bug where the quiet dependency flag didn't silence warnings in some

lib/src/parse/stylesheet.dart

@@ -1120,7 +1120,9 @@ abstract class StylesheetParser extends Parser {
   String parseImportUrl(String url) {
     // Backwards-compatibility for implementations that allow absolute Windows
     // paths in imports.
-    if (p.windows.isAbsolute(url)) return p.windows.toUri(url).toString();
+    if (p.windows.isAbsolute(url) && !p.url.isRootRelative(url)) {
+      return p.windows.toUri(url).toString();
+    }
 
     // Throw a [FormatException] if [url] is invalid.
     Uri.parse(url);

test/dart_api/importer_test.dart

@@ -93,6 +93,27 @@ void main() {
     '''));
   });
 
+  group("the imported URL", () {
+    // Regression test for #1137.
+    test("isn't changed if it's root-relative", () {
+      compileString('@import "/orange";', importers: [
+        TestImporter(expectAsync1((url) {
+          expect(url, equals(Uri.parse("/orange")));
+          return Uri.parse("u:$url");
+        }), (url) => ImporterResult('', syntax: Syntax.scss))
+      ]);
+    });
+
+    test("is converted to a file: URL if it's an absolute Windows path", () {
+      compileString('@import "C:/orange";', importers: [
+        TestImporter(expectAsync1((url) {
+          expect(url, equals(Uri.parse("file:///C:/orange")));
+          return Uri.parse("u:$url");
+        }), (url) => ImporterResult('', syntax: Syntax.scss))
+      ]);
+    });
+  });
+
   test("uses an importer's source map URL", () {
     late SingleMapping map;
     compileString('@import "orange";',

test/node_api/importer_test.dart

@@ -364,6 +364,25 @@ void main() {
           }))));
       expect(result.stats.includedFiles, equals(['foo']));
     });
+
+    // Regression test for #1137.
+    test("isn't changed if it's root-relative", () {
+      renderSync(RenderOptions(
+          data: "@import '/foo'",
+          importer: allowInterop(expectAsync2((url, _) {
+            expect(url, equals('/foo'));
+            return NodeImporterResult(contents: '');
+          }))));
+    });
+
+    test("is converted to a file: URL if it's an absolute Windows path", () {
+      renderSync(RenderOptions(
+          data: "@import 'C:/foo'",
+          importer: allowInterop(expectAsync2((url, _) {
+            expect(url, equals('file:///C:/foo'));
+            return NodeImporterResult(contents: '');
+          }))));
+    });
   });
 
   group("the previous URL", () {