Enable @extend.

Still tons of runtime errors.

lib/src/extend/extender.dart

@@ -89,7 +89,7 @@ class Extender {
     return new SelectorList(newList.where((complex) => complex != null));
   }
 
-  List<ComplexSelector> _extendComplex(ComplexSelector complex,
+  Iterable<ComplexSelector> _extendComplex(ComplexSelector complex,
       Map<SimpleSelector, Set<SelectorList>> extensions) {
     // This could be written more simply using [List.map], but we want to avoid
     // any allocations in the common case where no extends apply.
@@ -104,8 +104,10 @@ class Extender {
           if (changed) extendedNotExpanded.add([[component]]);
         } else {
           if (!changed) {
-            extendedNotExpanded =
-                complex.components.take(i).map((component) => [[component]]);
+            extendedNotExpanded = complex.components
+                .take(i)
+                .map((component) => [[component]])
+                .toList();
           }
           changed = true;
           extendedNotExpanded.add(extended);

lib/src/parser.dart

@@ -69,6 +69,12 @@ class Parser {
     return selector;
   }
 
+  SimpleSelector parseSimpleSelector() {
+    var simple = _simpleSelector();
+    _scanner.expectDone();
+    return simple;
+  }
+
   VariableDeclaration _variableDeclaration() {
     if (!_scanner.scanChar($dollar)) return null;
 

lib/src/visitor/sass/statement/perform.dart

@@ -66,6 +66,14 @@ class PerformVisitor extends StatementVisitor {
     _parent.addChild(new CssDeclaration(name, cssValue, span: node.span));
   }
 
+  void visitExtendRule(ExtendRule node) {
+    var targetText = _performInterpolation(node.selector);
+
+    // TODO: recontextualize parse errors.
+    var simple = new Parser(targetText.value).parseSimpleSelector();
+    _extender.addExtension(_selector.value, simple);
+  }
+
   void visitAtRule(AtRule node) {
     var value = node.value == null
         ? null