Validate dynamic children.

lib/src/ast/css/media_rule.dart

@@ -13,7 +13,6 @@ class CssMediaRule extends CssParentNode {
 
   final FileSpan span;
 
-  // TODO: validate that children contains only at-rule and style rule nodes?
   CssMediaRule(this.queries, {this.span});
 
   /*=T*/ accept/*<T>*/(CssVisitor/*<T>*/ visitor) =>

lib/src/ast/css/node.dart

@@ -38,7 +38,6 @@ abstract class CssParentNode extends CssNode {
         children = new UnmodifiableListView<CssNode>(children);
 
   void addChild(CssNode child) {
-    // TODO: validate that children are valid?
     child._parent = this;
     child._indexInParent = _children.length;
     _children.add(child);

lib/src/ast/sass/statement/at_rule.dart

@@ -17,8 +17,6 @@ class AtRule implements Statement {
 
   final FileSpan span;
 
-  // TODO: validate that children only contains variable, at-rule, declaration,
-  // or style nodes?
   AtRule(this.name, {this.value, Iterable<Statement> children, this.span})
       : children = children == null ? null : new List.unmodifiable(children);
 

lib/src/ast/sass/statement/style_rule.dart

@@ -15,8 +15,6 @@ class StyleRule implements Statement {
 
   final FileSpan span;
 
-  // TODO: validate that children only contains variable, at-rule, declaration,
-  // or style nodes?
   StyleRule(this.selector, Iterable<Statement> children, {this.span})
       : children = new List.unmodifiable(children);
 

lib/src/visitor/perform.dart

@@ -98,6 +98,10 @@ class PerformVisitor extends StatementVisitor
   }
 
   void visitExtendRule(ExtendRule node) {
+    if (_selector == null || _declarationName != null) {
+      throw node.span.message("@extend may only be used within style rules.");
+    }
+
     var targetText = _interpolationToValue(node.selector);
 
     // TODO: recontextualize parse errors.
@@ -107,6 +111,11 @@ class PerformVisitor extends StatementVisitor
   }
 
   void visitAtRule(AtRule node) {
+    if (_declarationName != null) {
+      throw node.span.message(
+          "At-rules may not be used within nested declarations.");
+    }
+
     var value = node.value == null
         ? null
         : _interpolationToValue(node.value, trim: true);
@@ -171,6 +180,11 @@ class PerformVisitor extends StatementVisitor
   }
 
   void visitMediaRule(MediaRule node) {
+    if (_declarationName != null) {
+      throw node.span.message(
+          "Media rules may not be used within nested declarations.");
+    }
+
     var queryIterable = node.queries.map(_visitMediaQuery);
     var queries = _mediaQueries == null
         ? new List<CssMediaQuery>.unmodifiable(queryIterable)
@@ -224,6 +238,11 @@ class PerformVisitor extends StatementVisitor
   Value visitReturn(Return node) => node.expression.accept(this);
 
   void visitStyleRule(StyleRule node) {
+    if (_declarationName != null) {
+      throw node.span.message(
+          "Style rules may not be used within nested declarations.");
+    }
+
     var selectorText = _interpolationToValue(node.selector, trim: true);
     var parsedSelector = new Parser(selectorText.value).parseSelector();