fix ClassTypeAlias extends

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 269 matching lines @@
   @override
   JS.Expression visitTypeName(TypeName node) => _emitTypeName(node.type);
 
   @override
   JS.Statement visitClassTypeAlias(ClassTypeAlias node) {
     // If we've already emitted this class, skip it.
     var type = node.element.type;
     if (_pendingClasses.remove(node.element) == null) return null;
 
     var name = node.name.name;
-    var heritage =
-        js.call('dart.mixin(#)', [_visitList(node.withClause.mixinTypes)]);
-    var classDecl = new JS.ClassDeclaration(
-        new JS.ClassExpression(new JS.Identifier(name), heritage, []));
+    var classDecl = new JS.ClassDeclaration(new JS.ClassExpression(
+        new JS.Identifier(name), _classHeritage(node), []));
 
     return _finishClassDef(type, classDecl);
   }
 
   JS.Statement _emitJsType(String dartClassName, DartObjectImpl jsName) {
     var jsTypeName = getConstantField(jsName, 'name', types.stringType);
 
     if (jsTypeName != null && jsTypeName != dartClassName) {
       // We export the JS type as if it was a Dart type. For example this allows
       // `dom.InputElement` to actually be HTMLInputElement.
@@ skipping 226 matching lines @@
     var typeParams = type.typeParameters.map((p) => p.name);
     if (isPublic(name)) _exports.add(genericName);
     return js.statement('let # = dart.generic(function(#) { #; return #; });', [
       genericName,
       typeParams,
       body,
       name
     ]);
   }
 
-  JS.Expression _classHeritage(ClassDeclaration node) {
+  JS.Expression _classHeritage(node) {

[vsm, 2015/04/24 22:31:55]

node could still be typed as AstNode, right?

[Jennifer Messerly, 2015/04/24 23:06:13]

it breaks the .element and .withClause.

     if (node.element.type.isObject) return null;
 
-    JS.Expression heritage = null;
-    if (node.extendsClause != null) {
-      heritage = _visit(node.extendsClause.superclass);
+    DartType supertype;
+    if (node is ClassDeclaration) {
+      var ext = node.extendsClause;
+      supertype = ext != null ? ext.superclass.type : types.objectType;
     } else {
-      heritage = _emitTypeName(types.objectType);
+      supertype = (node as ClassTypeAlias).superclass.type;
     }
+
+    JS.Expression heritage = _emitTypeName(supertype);
+
     if (node.withClause != null) {
       var mixins = _visitList(node.withClause.mixinTypes);
       mixins.insert(0, heritage);
       heritage = js.call('dart.mixin(#)', [mixins]);
     }
+
     return heritage;
   }
 
   List<JS.Method> _emitClassMethods(ClassDeclaration node,
       List<ConstructorDeclaration> ctors, List<FieldDeclaration> fields) {
     var element = node.element;
     var type = element.type;
     var isObject = type.isObject;
     var name = node.name.name;
 
@@ skipping 1944 matching lines @@
 // TODO(jmesserly): validate the library. See issue #135.
 bool _isJsNameAnnotation(DartObjectImpl value) => value.type.name == 'JsName';
 
 bool _isJsPeerInterface(DartObjectImpl value) =>
     value.type.name == 'JsPeerInterface';
 
 // TODO(jacobr): we would like to do something like the following
 // but we don't have summary support yet.
 // bool _supportJsExtensionMethod(AnnotatedNode node) =>
 //    _getAnnotation(node, "SupportJsExtensionMethod") != null;