Check extend usage after the fact.

This means that extensions that match but fail to unify are valid, but
we think that's okay; see sass/sass#2250.

README.md

@@ -180,6 +180,9 @@ Sass to update the reference behavior.
 
 14. Universal selector unification is symmetrical. See [issue 2247][].
 
+15. `@extend` doesn't produce an error if it matches but fails to unify. See
+    [issue 2250][].
+
 [issue 1599]: https://github.com/sass/sass/issues/1599
 [issue 1126]: https://github.com/sass/sass/issues/1126
 [issue 2120]: https://github.com/sass/sass/issues/2120
@@ -194,5 +197,6 @@ Sass to update the reference behavior.
 [issue 2245]: https://github.com/sass/sass/issues/2245
 [issue 303]: https://github.com/sass/sass/issues/303
 [issue 2247]: https://github.com/sass/sass/issues/2247
+[issue 2250]: https://github.com/sass/sass/issues/2250
 
 Disclaimer: this is not an official Google product.

lib/src/extend/extender.dart

@@ -173,14 +173,12 @@ class Extender {
     if (rules == null) return;
     if (newExtenders == null) return;
     for (var rule in rules) {
-      try {
-        var extended = _extendList(
-            rule.selector.value, {target: newExtenders}, _mediaContexts[rule]);
-
-        // A selector can't extend its own rule. We still have to run the extend
-        // to set [state.isUsed] appropriately, though.
+      // A selector can't extend its own rule.
       if (identical(rule.selector.value, extender)) continue;
-        rule.selector.value = extended;
+
+      try {
+        rule.selector.value = _extendList(
+            rule.selector.value, {target: newExtenders}, _mediaContexts[rule]);
       } on SassException catch (error) {
         throw new SassException(
             "From ${rule.selector.span.message('')}\n"
@@ -195,13 +193,15 @@ class Extender {
   /// Throws a [SassException] if any (non-optional) extensions failed to match
   /// any selectors.
   void finalize() {
-    _extensions.forEach((target, sources) {
-      sources.forEach((_, state) {
-        if (state.isOptional || state.isUsed) return;
+    _extensions.forEach((target, extensions) {
+      if (_selectors.containsKey(target)) return;
+
+      extensions.forEach((_, extension) {
+        if (extension.isOptional) return;
         throw new SassException(
             'The target selector was not found.\n'
             'Use "@extend $target !optional" to avoid this error.',
-            state.span);
+            extension.span);
       });
     });
   }
@@ -343,7 +343,6 @@ class Extender {
     // need any unification.
     if (options.length == 1) {
       return options.first.map((state) {
-        state.isUsed = true;
         state.assertCompatibleMediaContext(mediaQueryContext);
         return state.extender;
       }).toList();
@@ -416,7 +415,6 @@ class Extender {
       var specificity = _sourceSpecificityFor(compound);
       for (var state in path) {
         state.assertCompatibleMediaContext(mediaQueryContext);
-        state.isUsed = true;
         lineBreak = lineBreak || state.extender.lineBreak;
         specificity = math.max(specificity, state.specificity);
       }

lib/src/extend/extension.dart

@@ -29,9 +29,6 @@ class Extension {
   /// originally in the document, rather than one defined with `@extend`.
   final bool isOriginal;
 
-  /// Whether this extension matched a selector.
-  var isUsed = false;
-
   /// The media query context to which this extend is restricted, or `null` if
   /// it can apply within any context.
   List<CssMediaQuery> get mediaContext => _mediaContext;