Move emptiness-checking to the serializer.

We can't actually know if a rule will be empty until we've evaluated the
entire stylesheet, since it might contain a placeholder that may or may
not be extended.

lib/src/ast/css/at_rule.dart

@@ -17,6 +17,8 @@ class CssAtRule extends CssParentNode {
 
   final FileSpan span;
 
+  bool get isInvisible => false;
+
   CssAtRule(this.name, this.span, {bool childless: false, this.value})
       : isChildless = childless;
 

lib/src/ast/css/node.dart

@@ -14,6 +14,8 @@ abstract class CssNode extends AstNode {
 
   int _indexInParent;
 
+  bool get isInvisible => false;
+
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor);
 
   void remove() {
@@ -35,6 +37,15 @@ abstract class CssParentNode extends CssNode {
   final List<CssNode> children;
   final List<CssNode> _children;
 
+  bool get isInvisible {
+    if (_isInvisible == null) {
+      _isInvisible = children.every((child) => child.isInvisible);
+    }
+    return _isInvisible;
+  }
+
+  bool _isInvisible;
+
   CssParentNode() : this._([]);
 
   CssParentNode._(List<CssNode> children)

lib/src/visitor/perform.dart

@@ -108,12 +108,6 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
         child.accept(this);
       }
     });
-
-    if (innerCopy == null) return;
-    while (innerCopy != root && innerCopy.children.isEmpty) {
-      innerCopy.remove();
-      innerCopy = innerCopy.parent;
-    }
   }
 
   CssParentNode _trimIncluded(List<CssParentNode> nodes) {
@@ -245,7 +239,7 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
           for (var child in node.children) {
             child.accept(this);
           }
-        }, removeIfEmpty: true);
+        });
       }
     }, through: (node) => node is CssStyleRule);
   }
@@ -371,12 +365,10 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
             for (var child in node.children) {
               child.accept(this);
             }
-          }, removeIfEmpty: true);
+          });
         }
       });
-    },
-        through: (node) => node is CssStyleRule || node is CssMediaRule,
-        removeIfEmpty: true);
+    }, through: (node) => node is CssStyleRule || node is CssMediaRule);
   }
 
   List<CssMediaQuery> _mergeMediaQueries(
@@ -427,7 +419,7 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
           child.accept(this);
         }
       });
-    }, through: (node) => node is CssStyleRule, removeIfEmpty: true);
+    }, through: (node) => node is CssStyleRule);
   }
 
   void visitSupportsRule(SupportsRule node) {
@@ -453,7 +445,7 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
           for (var child in node.children) {
             child.accept(this);
           }
-        }, removeIfEmpty: true);
+        });
       }
     }, through: (node) => node is CssStyleRule);
   }
@@ -770,8 +762,7 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
   /*=T*/ _withParent/*<S extends CssParentNode, T>*/(
       /*=S*/ node,
       /*=T*/ callback(),
-      {bool through(CssNode node),
-      bool removeIfEmpty: false}) {
+      {bool through(CssNode node)}) {
     var oldParent = _parent;
 
     // Go up through parents that match [through].
@@ -785,7 +776,6 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
     parent.addChild(node);
     _parent = node;
     var result = _environment.scope(callback);
-    if (removeIfEmpty && node.children.isEmpty) node.remove();
     _parent = oldParent;
 
     return result;

lib/src/visitor/serialize.dart

@@ -50,6 +50,7 @@ class _SerializeCssVisitor
 
   void visitStylesheet(CssStylesheet node) {
     for (var child in node.children) {
+      if (child.isInvisible) continue;
       child.accept(this);
       _buffer.writeln();
     }
@@ -394,7 +395,7 @@ class _SerializeCssVisitor
 
   void _visitChildren(Iterable<CssNode> children) {
     _buffer.writeCharCode($lbrace);
-    if (children.isEmpty) {
+    if (children.every((child) => child.isInvisible)) {
       _buffer.writeCharCode($rbrace);
       return;
     }
@@ -402,6 +403,7 @@ class _SerializeCssVisitor
     _buffer.writeln();
     _indent(() {
       for (var child in children) {
+        if (child.isInvisible) continue;
         child.accept(this);
         _buffer.writeln();
       }