Fix for importers that return null in load (#1139)

CHANGELOG.md

@@ -3,6 +3,11 @@
 * Fix a bug where `@at-root (without: all)` wouldn't properly remove a
   `@keyframes` context when parsing selectors.
 
+### Dart API
+
+* Fix a bug that prevented importers from returning null when loading from a
+  URL that they had already canonicalized.
+
 ## 1.29.0
 
 * Support a broader syntax for `@supports` conditions, based on the latest

lib/src/import_cache.dart

@@ -162,6 +162,7 @@ Relative canonical URLs are deprecated and will eventually be disallowed.
         baseImporter: baseImporter, baseUrl: baseUrl, forImport: forImport);
     if (tuple == null) return null;
     var stylesheet = importCanonical(tuple.item1, tuple.item2, tuple.item3);
+    if (stylesheet == null) return null;
     return Tuple2(tuple.item1, stylesheet);
   }
 

test/dart_api/importer_test.dart

@@ -168,4 +168,17 @@ void main() {
       return true;
     })));
   });
+
+  test("avoids importer when only load() returns null", () {
+    expect(() {
+      compileString('@import "orange";', importers: [
+        TestImporter((url) => Uri.parse("u:$url"), (url) => null)
+      ]);
+    }, throwsA(predicate((error) {
+      expect(error, const TypeMatcher<SassException>());
+      expect(error.toString(),
+          startsWith("Error: Can't find stylesheet to import"));
+      return true;
+    })));
+  });
 }