Support for @mixin, @include and @extend

pkg/csslib/lib/src/tree_printer.dart

Index: pkg/csslib/lib/src/tree_printer.dart
diff --git a/pkg/csslib/lib/src/tree_printer.dart b/pkg/csslib/lib/src/tree_printer.dart
index 24cca64f1fd741c168c37853f9186d88c781a21b..ce528409c2730031bce352e44f5de289ba7c0744 100644
--- a/pkg/csslib/lib/src/tree_printer.dart
+++ b/pkg/csslib/lib/src/tree_printer.dart
@@ -141,7 +141,38 @@ class _TreePrinter extends Visitor {
 
   void visitVarDefinitionDirective(VarDefinitionDirective node) {
     heading('Less variable definition', node);
+    output.depth++;
     visitVarDefinition(node.def);
+    output.depth--;
+  }
+
+  void visitMixinRulesetDirective(MixinRulesetDirective node) {
+    heading('Mixin top-level ${node.name}', node);
+    output.depth++;
+    _visitNodeList(node.rulesets);
+    output.depth--;
+  }
+
+  void visitMixinDeclarationDirective(MixinDeclarationDirective node) {
+    heading('Mixin declaration ${node.name}', node);
+    output.depth++;
+    visitDeclarationGroup(node.declarations);
+    output.depth--;
+  }
+
+  /**
+   * Added optional newLine for handling @include at top-level vs/ inside of
+   * a declaration group.
+   */
+  void visitIncludeDirective(IncludeDirective node) {
+    heading('IncludeDirective ${node.name}', node);
+  }
+
+  void visitIncludeMixinAtDeclaration(IncludeMixinAtDeclaration node) {
+    heading('IncludeMixinAtDeclaration ${node.include.name}', node);
+    output.depth++;
+    visitIncludeDirective(node.include);
+    output.depth--;
   }
 
   void visitRuleSet(RuleSet node) {